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79,137,758

Common Stock Issuable [Member]

79,137,758

Stock Based Compensation Shares

^[1]

Common Stock Held In Treasury [Member]

14,645,901

15,068,009

14,835,256

13,933,232

Stock Based Compensation Shares

1,352,211

^[1]

(189,355)

(217,434)

Common Stock Outstanding [Member]

64,491,857

64,069,749

64,302,502

65,204,526

Stock Based Compensation Shares

(1,352,211)

^[1]

(189,355)

(217,434)

[1]	Represents net shares issued from treasury pursuant to the Company’s equity-based compensation programs.

Equity (Balance of and Changes in Each Component of AOCI) (Details) - USD ($)
$ in Thousands

6 Months Ended

Jun. 30, 2017

Jun. 30, 2016

Dec. 31, 2016

Dec. 31, 2015

Other Comprehensive Income (Loss), before Reclassifications, before Tax

$ 1,671,898

$ 1,985,374

$ 1,139,329

$ 708,284

760,556

1,853,044

Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax

(36,425)

(20,899)

Accumulated Translation Adjustment [Member]

191,562

555,055

Other Comprehensive Income (Loss), before Reclassifications, before Tax

(151,189)

(93,476)

(172,541)

(181,151)

(13,936)

99,374

Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax

Accumulated Net Unrealized Investment Gain (Loss) [Member]

35,288

(11,699)

Other Comprehensive Income (Loss), before Reclassifications, before Tax

^[1]

1,864,477

2,126,815

1,355,033

935,697

^[1]

774,688

1,759,753

Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax

^[1]

(39,360)

(24,366)

Accumulated Defined Benefit Plans Adjustment [Member]

^[1]

225,884

544,269

Other Comprehensive Income (Loss), before Reclassifications, before Tax

(41,390)

(47,965)

$ (43,163)

$ (46,262)

(196)

(6,083)

Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax

2,935

3,467

Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]

$ 966

$ (913)

[1]	Includes cash flow hedges of $1,131 and $(2,496) as of June 30, 2017 and December 31, 2016, respectively, and $(41,192) and $(29,397) as of June 30, 2016 and December 31, 2015, respectively. See Note 5 - “Derivative Instruments” for additional information on cash flow hedges.

Equity (Balance of and Changes in Each Component of AOCI) (Footnote) (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended		12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Mar. 31, 2016	Mar. 31, 2015	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	$ 1,131	$ (41,192)	$ 7,690	$ (21,794)	$ 1,131	$ (41,192)	$ (2,496)	$ (29,397)

Equity (Schedule of Reclassifications out of AOCI) (Details) - USD ($)
$ in Thousands

3 Months Ended

6 Months Ended

Jun. 30, 2017

Jun. 30, 2016

Jun. 30, 2017

Jun. 30, 2016

Investment related gains (losses), net

$ 59,696

$ 119,110

$ 137,408

$ 32,041

Investment income

518,538

507,666

1,032,902

924,932

Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest

339,315

353,223

547,159

460,803

Income Tax Expense (Benefit)

(107,125)

(117,120)

(169,457)

(148,228)

Unrealized Appreciation Depreciation Of Investments [Member]

232,190

236,103

377,702

312,575

Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]

Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest

45,122

34,603

39,360

24,366

Income Tax Expense (Benefit)

(15,218)

(9,646)

(12,024)

(4,996)

Unrealized Appreciation Depreciation Of Investments [Member] | Unrealized Gains Losses On Available For Sale Securities [Member]

29,904

24,957

27,336

19,370

Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]

Investment related gains (losses), net

40,374

30,190

28,517

11,899

Unrealized Appreciation Depreciation Of Investments [Member] | Interest Rate Swap [Member]

Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]

Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net

^[1]

132

329

253

Unrealized Appreciation Depreciation Of Investments [Member] | Forward Contracts [Member]

Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]

Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net

^[1]

(1,045)

101

(257)

Unrealized Appreciation Depreciation Of Investments [Member] | DAC Attributed To Unrealized Gains Losses [Member]

Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]

Deferred policy acquisition costs attributed to unrealized gains and losses

4,565

^[1]

5,365

^[1]

(10,413)

^[2]

12,471

^[2]

Amortization Of Unrealized Pension And Postretirement Benefits [Member]

Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]

Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest

(1,479)

(1,916)

(2,935)

(3,467)

Income Tax Expense (Benefit)

517

670

1,027

1,213

Amounts Reclassified From AOCI [Member]

(962)

(1,246)

(1,908)

(2,254)

Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]

Prior Service Cost [Member] | Amortization Of Unrealized Pension And Postretirement Benefits [Member]

28,942

23,711

25,428

17,116

Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]

Prior service cost

^[2]

(75)

^[2]

142

^[3]

(153)

^[3]

Actuarial Gains Losses [Member] | Amortization Of Unrealized Pension And Postretirement Benefits [Member]

Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]

Actuarial gains/(losses)

$ (1,539)

^[2]

$ (1,841)

^[2]

$ 3,077

^[3]

$ (3,314)

^[3]

[1]	See Note 5 - “Derivative Instruments” for additional information on cash flow hedges.
[2]	This AOCI component is included in the computation of the deferred policy acquisition cost. See Note 8 – “Deferred Policy Acquisition Costs” of the 2016 Annual Report for additional details.
[3]	This AOCI component is included in the computation of the net periodic pension cost. See Note 10 – “Employee Benefit Plans” for additional details.

Equity (Narrative) (Details) $ in Millions	3 Months Ended
Equity (Narrative) (Details) $ in Millions	Jun. 30, 2017 USD ($) shares
Equity [Abstract]
Stock Repurchase Program, Authorized Amount \| $	$ 400.0
Treasury Stock, Shares, Acquired \| shares	0

Equity Based Compensation (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Equity compensation expense	$ 11.4	$ 18.9
Stock appreciation rights granted	200,000
Stock appreciation rights granted, weighted average exercise price (per share)	$ 129.72
Performance contingent units	200,000
Shares Granted To Non Employee Directors	8,177
Options exercisable (shares)	1,700,000
Options exercisable, weighted average price (per share)	$ 60.31
Options, remaining term	4 years 9 months
Compensation cost not yet recognized	$ 38.6
Period for recognition	1 year 5 months

Investments (Investments in Fixed Maturity and Equity Securities Available-for-Sale by Sector) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 21,252,180	$ 18,924,711
Estimated Fair Value	$ 22,341,661	$ 19,619,084
% of Total	100.00%	100.00%
Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 33,738,334	$ 30,211,787
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	2,779,535	2,243,037
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	172,443	361,199
Estimated Fair Value	$ 36,345,426	$ 32,093,625
% of Total	100.00%	100.00%
Other-than-temporary impairments in AOCI	$ 275	$ 100
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	109,958	285,579
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	957	3,440
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	6,638	13,658
Estimated Fair Value	$ 104,277	$ 275,361
% of Total	100.00%	100.00%
Corporate Debt Securities [Member] \| Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 21,252,180	$ 18,924,711
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	1,189,750	911,618
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	100,269	217,245
Estimated Fair Value	$ 22,341,661	$ 19,619,084
% of Total	61.50%	61.10%
Other-than-temporary impairments in AOCI	$ 0	$ 0
Canadian and Canadian provincial governments \| Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,713,972	2,561,605
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	1,296,242	1,085,982
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	2,460	3,541
Estimated Fair Value	$ 4,007,754	$ 3,644,046
% of Total	11.00%	11.40%
Other-than-temporary impairments in AOCI	$ 0	$ 0
Residential mortgage-backed securities \| Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,505,474	1,258,039
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	42,619	33,917
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	8,794	13,380
Estimated Fair Value	$ 1,539,299	$ 1,278,576
% of Total	4.20%	4.00%
Other-than-temporary impairments in AOCI	$ 0	$ 375
Asset-backed securities \| Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,630,499	1,443,822
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	17,266	9,350
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	5,924	23,828
Estimated Fair Value	$ 1,641,841	$ 1,429,344
% of Total	4.50%	4.50%
Other-than-temporary impairments in AOCI	$ 275	$ 275
Commercial Mortgage Backed Securities [Member] \| Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,558,035	1,342,440
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	28,928	28,973
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	4,935	7,759
Estimated Fair Value	$ 1,582,028	$ 1,363,654
% of Total	4.40%	4.20%
Other-than-temporary impairments in AOCI	$ 0	$ 0
US Treasury and Government [Member] \| Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,738,419	1,518,702
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	15,193	12,644
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	32,048	63,044
Estimated Fair Value	$ 1,721,564	$ 1,468,302
% of Total	4.70%	4.60%
Other-than-temporary impairments in AOCI	$ 0	$ 0
US States and Political Subdivisions Debt Securities [Member] \| Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	599,622	566,761
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	47,564	37,499
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	8,216	12,464
Estimated Fair Value	$ 638,970	$ 591,796
% of Total	1.80%	1.80%
Other-than-temporary impairments in AOCI	$ 0	$ 0
Foreign Government Debt Securities [Member] \| Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,740,133	2,595,707
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	141,973	123,054
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	9,797	19,938
Estimated Fair Value	$ 2,872,309	$ 2,698,823
% of Total	7.90%	8.40%
Other-than-temporary impairments in AOCI	$ 0	$ 0
Nonredeemable Preferred Stock [Member] \| Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	34,545	55,812
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	435	1,648
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	3,021	6,337
Estimated Fair Value	$ 31,959	$ 51,123
% of Total	30.60%	18.60%
Other equity securities \| Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 75,413	$ 229,767
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	522	1,792
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	3,617	7,321
Estimated Fair Value	$ 72,318	$ 224,238
% of Total	69.40%	81.40%

Investments (Fixed Maturity Securities Pledged And Received As Collateral) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 21,252,180	$ 18,924,711
Available-for-sale Securities	22,341,661	19,619,084
Collateral Fixed Maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	69,849	207,066
Available-for-sale Securities	74,441	210,676
Collateral Received Fixed Maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	457,801	300,925
Reported Value Measurement [Member]
Schedule of Available-for-sale Securities [Line Items]
Assets Held-in-trust	14,706,225	12,135,258
Estimate of Fair Value Measurement [Member]
Schedule of Available-for-sale Securities [Line Items]
Assets Held-in-trust	$ 15,723,178	$ 12,874,370

Investments (Exposure To Concentrations Of Credit Risk) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 21,252,180	$ 18,924,711
Available-for-sale Securities	22,341,661	19,619,084
Canadian Province Of Quebec Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	886,518	832,764
Available-for-sale Securities	1,228,391	1,126,433
Canadian Province Of Ontario Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,060,922	1,004,261
Available-for-sale Securities	$ 1,785,573	$ 1,612,957

Investments (Amortized Cost and Fair Value of Fixed Maturities by Contractual Maturities) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Available For Sale Securities Debt Maturities Amortized Cost
Due in one year or less	$ 861,051
Due after one year through five years	7,421,897
Due after five year through ten years	9,514,341
Due after ten years	11,247,037
Asset and mortgage-backed securities	4,694,008
Total	33,738,334	$ 30,211,787
Available For Sale Securities Debt Maturities Fair Value
Due in one year or less	868,405
Due after one year through five years	7,696,337
Due after five year through ten years	10,016,984
Due after ten years	13,000,532
Asset and mortgage-backed securities	4,763,168
Total	$ 36,345,426	$ 32,093,625

Investments (Amortized Cost and Fair Value of Fixed Maturity Holdings by Industry Type) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	$ 21,252,180	$ 18,924,711
Estimated Fair Value	$ 22,341,661	$ 19,619,084
% of Total	100.00%	100.00%
Financial Services Sector [Member]
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	$ 7,741,482	$ 6,725,199
Estimated Fair Value	$ 8,077,195	$ 6,888,968
% of Total	36.10%	35.20%
Commercial and Industrial Sector [Member]
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	$ 11,215,292	$ 10,228,813
Estimated Fair Value	$ 11,792,664	$ 10,639,613
% of Total	52.90%	54.20%
Utility Sector [Member]
Corporate Securities by Investment Sector And Credit Rating [Line Items]
Amortized Cost	$ 2,295,406	$ 1,970,699
Estimated Fair Value	$ 2,471,802	$ 2,090,503
% of Total	11.00%	10.60%

Investments (Credit Losses Recognized in Earnings on Fixed Maturity Securities Held by the Company) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Balance, beginning of period	$ 3,677	$ 7,284	$ 6,013	$ 7,284
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Reductions, Change in Status	0	(310)	(2,336)	(310)
Balance, end of period	$ 3,677	$ 6,974	$ 3,677	$ 6,974

Investments (Fair Value Below Amortized Cost of Gross Unrealized Losses for Fixed Maturity and Equity Securities) (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Gross unrealized losses with number of securities [Line Items]
Gross Unrealized Losses	$ 179,081	$ 374,857
% of Total	100.00%	100.00%
Less than 20%
Gross unrealized losses with number of securities [Line Items]
Gross Unrealized Losses	$ 150,762	$ 337,831
% of Total	84.20%	90.10%
20% or more for less than six months
Gross unrealized losses with number of securities [Line Items]
Gross Unrealized Losses	$ 7,593	$ 19,438
% of Total	4.20%	5.20%
20% or more for six months or greater
Gross unrealized losses with number of securities [Line Items]
Gross Unrealized Losses	$ 20,726	$ 17,588
% of Total	11.60%	4.70%

Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Fixed maturity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	$ 6,362,753	$ 8,327,843
Gross Unrealized Losses - Less than 12 months	111,152	253,005
Estimated Fair Value - 12 months or greater	881,805	1,210,148
Gross Unrealized Losses - 12 months or greater	61,291	108,194
Estimated Fair Value	7,244,558	9,537,991
Gross Unrealized Losses	172,443	361,199
Equity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	64,990	212,899
Gross Unrealized Losses - Less than 12 months	3,617	7,851
Estimated Fair Value - 12 months or greater	24,807	28,630
Gross Unrealized Losses - 12 months or greater	3,021	5,807
Estimated Fair Value	89,797	241,529
Gross Unrealized Losses	6,638	13,658
Non-investment grade securities \| Fixed maturity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	295,307	374,831
Gross Unrealized Losses - Less than 12 months	4,837	10,607
Estimated Fair Value - 12 months or greater	118,570	215,908
Gross Unrealized Losses - 12 months or greater	27,347	45,409
Estimated Fair Value	413,877	590,739
Gross Unrealized Losses	32,184	56,016
Investment grade securities \| Fixed maturity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	6,067,446	7,953,012
Gross Unrealized Losses - Less than 12 months	106,315	242,398
Estimated Fair Value - 12 months or greater	763,235	994,240
Gross Unrealized Losses - 12 months or greater	33,944	62,785
Estimated Fair Value	6,830,681	8,947,252
Gross Unrealized Losses	140,259	305,183
Corporate Debt Securities [Member] \| Non-investment grade securities \| Fixed maturity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	255,991	330,757
Gross Unrealized Losses - Less than 12 months	4,547	7,914
Estimated Fair Value - 12 months or greater	93,562	163,152
Gross Unrealized Losses - 12 months or greater	26,081	41,605
Estimated Fair Value	349,553	493,909
Gross Unrealized Losses	30,628	49,519
Corporate Debt Securities [Member] \| Investment grade securities \| Fixed maturity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	2,895,605	4,661,706
Gross Unrealized Losses - Less than 12 months	46,389	124,444
Estimated Fair Value - 12 months or greater	399,546	549,273
Gross Unrealized Losses - 12 months or greater	23,252	43,282
Estimated Fair Value	3,295,151	5,210,979
Gross Unrealized Losses	69,641	167,726
Canadian and Canadian provincial governments \| Non-investment grade securities \| Fixed maturity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	1,247
Gross Unrealized Losses - Less than 12 months	3
Estimated Fair Value - 12 months or greater	0
Gross Unrealized Losses - 12 months or greater	0
Estimated Fair Value	1,247
Gross Unrealized Losses	3
Canadian and Canadian provincial governments \| Investment grade securities \| Fixed maturity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	116,719	101,578
Gross Unrealized Losses - Less than 12 months	2,457	3,541
Estimated Fair Value - 12 months or greater	0	0
Gross Unrealized Losses - 12 months or greater	0	0
Estimated Fair Value	116,719	101,578
Gross Unrealized Losses	2,457	3,541
Residential mortgage-backed securities \| Non-investment grade securities \| Fixed maturity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	0	0
Gross Unrealized Losses - Less than 12 months	0	0
Estimated Fair Value - 12 months or greater	107	412
Gross Unrealized Losses - 12 months or greater	4	12
Estimated Fair Value	107	412
Gross Unrealized Losses	4	12
Residential mortgage-backed securities \| Investment grade securities \| Fixed maturity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	471,933	490,473
Gross Unrealized Losses - Less than 12 months	6,872	9,733
Estimated Fair Value - 12 months or greater	100,785	112,216
Gross Unrealized Losses - 12 months or greater	1,918	3,635
Estimated Fair Value	572,718	602,689
Gross Unrealized Losses	8,790	13,368
Asset-backed securities \| Non-investment grade securities \| Fixed maturity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	0	5,904
Gross Unrealized Losses - Less than 12 months	0	700
Estimated Fair Value - 12 months or greater	7,295	12,581
Gross Unrealized Losses - 12 months or greater	557	1,465
Estimated Fair Value	7,295	18,485
Gross Unrealized Losses	557	2,165
Asset-backed securities \| Investment grade securities \| Fixed maturity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	285,211	563,259
Gross Unrealized Losses - Less than 12 months	1,451	12,010
Estimated Fair Value - 12 months or greater	204,154	257,166
Gross Unrealized Losses - 12 months or greater	3,916	9,653
Estimated Fair Value	489,365	820,425
Gross Unrealized Losses	5,367	21,663
Commercial mortgage-backed securities \| Non-investment grade securities \| Fixed maturity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months		5,815
Gross Unrealized Losses - Less than 12 months		735
Estimated Fair Value - 12 months or greater		0
Gross Unrealized Losses - 12 months or greater		0
Estimated Fair Value		5,815
Gross Unrealized Losses		735
Commercial mortgage-backed securities \| Investment grade securities \| Fixed maturity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	352,867	368,465
Gross Unrealized Losses - Less than 12 months	4,897	6,858
Estimated Fair Value - 12 months or greater	2,195	10,853
Gross Unrealized Losses - 12 months or greater	38	166
Estimated Fair Value	355,062	379,318
Gross Unrealized Losses	4,935	7,024
US Government Agencies Debt Securities [Member] \| Investment grade securities \| Fixed maturity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	1,378,976	1,056,101
Gross Unrealized Losses - Less than 12 months	31,962	63,044
Estimated Fair Value - 12 months or greater	13,763	0
Gross Unrealized Losses - 12 months or greater	86	0
Estimated Fair Value	1,392,739	1,056,101
Gross Unrealized Losses	32,048	63,044
US States and Political Subdivisions Debt Securities [Member] \| Investment grade securities \| Fixed maturity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	125,465	187,194
Gross Unrealized Losses - Less than 12 months	5,098	9,396
Estimated Fair Value - 12 months or greater	13,558	13,635
Gross Unrealized Losses - 12 months or greater	3,118	3,068
Estimated Fair Value	139,023	200,829
Gross Unrealized Losses	8,216	12,464
Foreign Government Debt Securities [Member] \| Non-investment grade securities \| Fixed maturity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	38,069	32,355
Gross Unrealized Losses - Less than 12 months	287	1,258
Estimated Fair Value - 12 months or greater	17,606	39,763
Gross Unrealized Losses - 12 months or greater	705	2,327
Estimated Fair Value	55,675	72,118
Gross Unrealized Losses	992	3,585
Foreign Government Debt Securities [Member] \| Investment grade securities \| Fixed maturity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	440,670	524,236
Gross Unrealized Losses - Less than 12 months	7,189	13,372
Estimated Fair Value - 12 months or greater	29,234	51,097
Gross Unrealized Losses - 12 months or greater	1,616	2,981
Estimated Fair Value	469,904	575,333
Gross Unrealized Losses	8,805	16,353
Nonredeemable Preferred Stock [Member] \| Equity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	0	10,831
Gross Unrealized Losses - Less than 12 months	0	831
Estimated Fair Value - 12 months or greater	24,807	21,879
Gross Unrealized Losses - 12 months or greater	3,021	5,506
Estimated Fair Value	24,807	32,710
Gross Unrealized Losses	3,021	6,337
Other equity securities \| Equity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	64,990	202,068
Gross Unrealized Losses - Less than 12 months	3,617	7,020
Estimated Fair Value - 12 months or greater	0	6,751
Gross Unrealized Losses - 12 months or greater	0	301
Estimated Fair Value	64,990	208,819
Gross Unrealized Losses	$ 3,617	$ 7,321

Investments (Investment Income Net of Related Expenses) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	$ 537,583	$ 525,229	$ 1,070,617	$ 960,349
Investment expense	(19,045)	(17,563)	(37,715)	(35,417)
Investment income, net of related expenses	518,538	507,666	1,032,902	924,932
Available-for-sale Securities [Member]
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	355,735	323,592	680,235	636,007
Mortgage Loans on Real Estate [Member]
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	44,442	41,900	88,789	81,692
Policy loans
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	15,194	16,372	30,466	32,506
Funds withheld at interest
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	97,367	112,893	224,945	168,873
Short-term Investments [Member]
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	1,779	2,322	3,289	4,513
Other invested assets
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	$ 23,066	$ 28,150	$ 42,893	$ 36,758

Investments (Investment Related Gains (Losses) Net) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Investment Gains And Losses [Abstract]
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net	$ 3,401	$ 846	$ 20,590	$ 34,663
Gain on investment activity	(54,220)	(53,615)	(72,113)	(80,807)
Available-for-sale Securities, Realized Losses, Excluding Other than Temporary Impairments	(10,471)	(22,556)	(23,034)	(34,343)
Other impairment losses and change in mortgage loan provision	(6,675)	211	(6,774)	(1,849)
Derivatives and other, net	22,622	87,840	95,103	(12,574)
Total investment related gains (losses), net	$ 56,295	$ 118,264	$ 116,818	$ (2,622)

Investments (Borrowed Securities Repurchased Securities And Repurchased Reversed Repurchased Securities) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Securities Loaned, Fair Value of Collateral	$ 121,064	$ 73,625
Securities Received as Collateral	109,000	80,000
Securities for Reverse Repurchase Agreements	486,700	476,531
Securities Sold under Agreements to Repurchase, Collateral, Right to Reclaim Securities	517,871	515,200
Reported Value Measurement [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Borrowed	269,280	263,820
Securities Loaned, Fair Value of Collateral	117,217	74,389
Estimate of Fair Value Measurement [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Borrowed	284,083	279,186
Securities for Reverse Repurchase Agreements	$ 509,579	$ 499,891

Investments (Mortgage Loans by Property Type) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015
Mortgage Loans on Real Estate [Line Items]
Recorded investment	$ 4,114,032		$ 3,783,207
Percentage of total	100.00%		100.00%
Unamortized Loan Commitment and Origination Fees and Unamortized Discounts or Premiums	$ (1,389)		$ 0
Allowance for Loan and Lease Losses, Real Estate	(8,156)	$ (7,786)	(7,685)	$ (6,499)	$ (6,824)	$ (6,813)
Mortgage loans on real estate (net of allowances of $8,156 and $7,685)	4,104,487		3,775,522
Apartment
Mortgage Loans on Real Estate [Line Items]
Recorded investment	$ 483,811		$ 447,088
Percentage of total	11.80%		11.80%
Retail
Mortgage Loans on Real Estate [Line Items]
Recorded investment	$ 1,248,238		$ 1,179,936
Percentage of total	30.30%		31.20%
Office building
Mortgage Loans on Real Estate [Line Items]
Recorded investment	$ 1,402,369		$ 1,270,113
Percentage of total	34.10%		33.60%
Industrial
Mortgage Loans on Real Estate [Line Items]
Recorded investment	$ 809,309		$ 713,461
Percentage of total	19.70%		18.80%
Other commercial
Mortgage Loans on Real Estate [Line Items]
Recorded investment	$ 170,305		$ 172,609
Percentage of total	4.10%		4.60%

Investments (Mortgage Loans by Maturity Date) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Mortgage Loans on Real Estate [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 4,114,032	$ 3,783,207
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	100.00%	100.00%
Due within five years
Mortgage Loans on Real Estate [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 1,037,000	$ 822,073
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	25.20%	21.70%
Due after five years through ten years
Mortgage Loans on Real Estate [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 2,236,805	$ 2,099,559
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	54.40%	55.50%
Due after ten years
Mortgage Loans on Real Estate [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 840,227	$ 861,575
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	20.40%	22.80%

Investments (Mortgage Loans by Credit Quality Indicator) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 4,114,032	$ 3,783,207
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	100.00%	100.00%
LTV 0 to 59 Percent [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 2,074,075	$ 1,925,755
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	50.40%	50.80%
LTV 60 To 69 Percent [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 1,518,077	$ 1,292,466
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	36.90%	34.20%
LTV 70 To 79 Percent [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 435,410	$ 415,330
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	10.60%	11.00%
LTV 80 to 100 Percent [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 86,470	$ 149,656
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	2.10%	4.00%
Debt Service Coverage Ratio Range One [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 3,902,414	$ 3,601,817
Debt Service Coverage Ratio Range One [Member] \| LTV 0 to 59 Percent [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	2,009,905	1,859,640
Debt Service Coverage Ratio Range One [Member] \| LTV 60 To 69 Percent [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	1,453,061	1,257,788
Debt Service Coverage Ratio Range One [Member] \| LTV 70 To 79 Percent [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	377,744	370,092
Debt Service Coverage Ratio Range One [Member] \| LTV 80 to 100 Percent [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	61,704	114,297
Debt Service Coverage Ratio Range Two [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	138,017	120,296
Debt Service Coverage Ratio Range Two [Member] \| LTV 0 to 59 Percent [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	60,110	64,749
Debt Service Coverage Ratio Range Two [Member] \| LTV 60 To 69 Percent [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	57,332	34,678
Debt Service Coverage Ratio Range Two [Member] \| LTV 70 To 79 Percent [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	20,575	20,869
Debt Service Coverage Ratio Range Two [Member] \| LTV 80 to 100 Percent [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	0	0
Debt Service Coverage Ratio Range Three [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	73,601	61,094
Debt Service Coverage Ratio Range Three [Member] \| LTV 0 to 59 Percent [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	4,060	1,366
Debt Service Coverage Ratio Range Three [Member] \| LTV 60 To 69 Percent [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	7,684	0
Debt Service Coverage Ratio Range Three [Member] \| LTV 70 To 79 Percent [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	37,091	24,369
Debt Service Coverage Ratio Range Three [Member] \| LTV 80 to 100 Percent [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 24,766	$ 35,359

Investments (Age Analysis of Past Due Recorded Investments) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	$ 3,729	$ 0
Financing Receivable, Recorded Investment, Current	4,110,303	3,783,207
Loans Receivable, Gross, Commercial, Real Estate	4,114,032	3,783,207
Financing Receivables, 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	0	0
Financing Receivables, 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	3,729	0
Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	$ 0	$ 0

Investments (Mortgage Loans by Method of Evaluation and Related Valuation Allowances) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Mortgage Loans:
Total mortgage loans	$ 4,114,032	$ 3,783,207
Mortgage Loans on Real Estate, Commercial and Consumer, Net, (Investment Based Operations Presentation) [Abstract]
Mortgage loans on real estate (net of valuation allowances)	4,104,487	3,775,522
Commercial Real Estate Portfolio Segment [Member]
Mortgage Loans:
Evaluated individually for credit losses	2,077	2,216
Evaluated collectively for credit losses	4,111,955	3,780,991
Total mortgage loans	4,114,032	3,783,207
Valuation Allowances:
Specific for credit losses	0	0
Non-specifically identified credit losses	8,156	7,685
Total valuation allowances	$ 8,156	$ 7,685

Investments (Loan Valuation Allowance for Mortgage Loans) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Allowance For Loan And Lease Losses Roll Forward
Balance, Beginning of Period	$ 7,786	$ 6,824	$ 7,685	$ 6,813
Provision (release)	366	(325)	467	(314)
Allowance for Loan and Lease Losses, Foreign Currency Translation	4	0	4	0
Balance, End of Period	$ 8,156	$ 6,499	$ 8,156	$ 6,499

Investments (Impaired Mortgage Loans) (Details) - USD ($)
$ in Thousands

3 Months Ended

6 Months Ended

Jun. 30, 2017

Jun. 30, 2016

Jun. 30, 2017

Jun. 30, 2016

Dec. 31, 2016

Financing Receivable Impaired [Line Items]

Unpaid Principal Balance

$ 2,620

$ 2,758

Recorded Investment

2,077

2,216

Related Allowance

Carrying Value

2,077

2,216

Average Investment

^[1]

2,088

$ 8,625

2,131

$ 11,224

Interest Income

107

216

Impaired mortgage loans with valuation allowance recorded

Financing Receivable Impaired [Line Items]

Unpaid Principal Balance

^[1]

4,724

7,279

Interest Income

Impaired mortgage loans with no valuation allowance recorded

Financing Receivable Impaired [Line Items]

Unpaid Principal Balance

2,620

2,758

Recorded Investment

2,077

2,216

Related Allowance

Carrying Value

2,077

$ 2,216

Average Investment

^[1]

2,088

3,901

2,131

3,945

Interest Income

$ 33

$ 107

$ 67

$ 216

[1]	Average recorded investment represents the average loan balances as of the beginning of period and all subsequent quarterly end of period balances.

Investments (Other Invested Assets) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Other invested assets	$ 1,498,370	$ 1,591,940
Equity securities
Other invested assets	104,277	275,361
Limited Partner [Member]
Other invested assets	746,573	687,522
Derivatives
Other invested assets	158,048	229,108
Fair Value Option Contractholder Directed Unit Linked Investments [Member]
Other invested assets	204,630	190,120
Other
Other invested assets	$ 284,842	$ 209,829

Investments (Narrative) (Details) $ in Thousands	3 Months Ended		6 Months Ended
Investments (Narrative) (Details) $ in Thousands	Jun. 30, 2017 USD ($) securities	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) securities	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) securities
Investment Narrative [Line Items]
Securities Sold under Agreements to Repurchase, Collateral, Right to Reclaim Cash	$ 47,900		$ 47,900		$ 28,800
Net Amount Assets Liabilities From Repurchase Reverse Repurchase Agreements	5,100		5,100		$ 5,500
Proceeds From Sale Of Available For Sale Securities Sold At Loss	710,500	$ 343,300	1,286,700	$ 585,800
Available-for-sale Securities, Realized Losses, Excluding Other than Temporary Impairments	$ 10,471	$ 22,556	$ 23,034	$ 34,343
Policy Loans Percent Of Total Investments	2.80%		2.80%		3.20%
Mortgage Loan Percent Of Cash And Invested Assets	8.30%		8.30%		8.40%
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions \| securities	1,134		1,134		1,535
Securities As Percentage Of Consolidated Stockholder Equity	10.00%		10.00%
Securities Borrowed Or Pledged, Minimum Fair Value Percentage			100.00%
Collateral Obtained Percent Of Fair Value Reverse Repurchase Agreements			100.00%
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	100.00%		100.00%		100.00%
Funds Withheld At Interest Percent Of Total Investments	12.10%		12.10%		13.10%
Funds withheld at interest	$ 5,968,856		$ 5,968,856		$ 5,875,919
Funds Held Under Reinsurance Agreements Asset Specific Client	$ 4,000,000		$ 4,000,000
Other Invested Assets Percent Of Cash And Invested Assets	3.00%		3.00%		3.60%
CALIFORNIA
Investment Narrative [Line Items]
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	22.00%		22.00%		22.10%

Derivative Instruments (Gross Notional Amount and Fair Value of Derivatives Contracts) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Derivative [Line Items]
Notional Amount	$ 14,561,937	$ 14,012,598
Carrying Value / Fair Value of Assets	281,527	283,094
Derivative Liability, Fair Value, Gross Liability	1,015,305	1,061,408
Not Designated as Hedging Instrument
Derivative [Line Items]
Notional Amount	13,079,853	12,649,093
Carrying Value / Fair Value of Assets	193,942	150,834
Derivative Liability, Fair Value, Gross Liability	981,851	1,029,451
Not Designated as Hedging Instrument \| Interest Rate Swap [Member]
Derivative [Line Items]
Notional Amount	973,825	949,556
Carrying Value / Fair Value of Assets	63,370	78,405
Derivative Liability, Fair Value, Gross Liability	4,375	5,949
Not Designated as Hedging Instrument \| Financial Futures
Derivative [Line Items]
Notional Amount	477,407	475,968
Carrying Value / Fair Value of Assets	0	0
Derivative Liability, Fair Value, Gross Liability	0	0
Not Designated as Hedging Instrument \| Foreign Currency Forwards
Derivative [Line Items]
Notional Amount	15,000	25,000
Carrying Value / Fair Value of Assets	21	0
Derivative Liability, Fair Value, Gross Liability	63	5,070
Not Designated as Hedging Instrument \| Consumer Price Index Swap [Member]
Derivative [Line Items]
Notional Amount	21,991	20,615
Carrying Value / Fair Value of Assets	0	0
Derivative Liability, Fair Value, Gross Liability	187	262
Not Designated as Hedging Instrument \| Credit Default Swaps
Derivative [Line Items]
Notional Amount	945,000	926,000
Carrying Value / Fair Value of Assets	7,620	12,012
Derivative Liability, Fair Value, Gross Liability	1,043	2,871
Not Designated as Hedging Instrument \| Equity Options
Derivative [Line Items]
Notional Amount	541,532	525,894
Carrying Value / Fair Value of Assets	28,301	33,459
Derivative Liability, Fair Value, Gross Liability	0	0
Not Designated as Hedging Instrument \| Longevity Swap [Member]
Derivative [Line Items]
Notional Amount	914,080	841,360
Carrying Value / Fair Value of Assets	33,349	26,958
Derivative Liability, Fair Value, Gross Liability	0	0
Not Designated as Hedging Instrument \| Mortality Swap [Member]
Derivative [Line Items]
Notional Amount	50,000	50,000
Carrying Value / Fair Value of Assets	0	0
Derivative Liability, Fair Value, Gross Liability	1,552	2,462
Not Designated as Hedging Instrument \| Embedded Derivatives In Modified Coinsurance Or Funds Withheld Arrangements
Derivative [Line Items]
Notional Amount	0	0
Carrying Value / Fair Value of Assets	61,281	0
Derivative Liability, Fair Value, Gross Liability	0	22,529
Not Designated as Hedging Instrument \| Embedded Derivatives In Indexed Annuity [Member]
Derivative [Line Items]
Notional Amount	0	0
Carrying Value / Fair Value of Assets	0	0
Derivative Liability, Fair Value, Gross Liability	812,718	805,672
Not Designated as Hedging Instrument \| Embedded Derivatives In Variable Annuity [Member]
Derivative [Line Items]
Notional Amount	0	0
Carrying Value / Fair Value of Assets	0	0
Derivative Liability, Fair Value, Gross Liability	161,913	184,636
Not Designated as Hedging Instrument \| Synthetic Guaranteed Investment Contracts
Derivative [Line Items]
Notional Amount	9,141,018	8,834,700
Carrying Value / Fair Value of Assets	0	0
Derivative Liability, Fair Value, Gross Liability	0	0
Designated As Hedging Instrument
Derivative [Line Items]
Notional Amount	1,482,084	1,363,505
Carrying Value / Fair Value of Assets	87,585	132,260
Derivative Liability, Fair Value, Gross Liability	33,454	31,957
Designated As Hedging Instrument \| Interest Rate Swap [Member]
Derivative [Line Items]
Notional Amount	435,000	435,000
Carrying Value / Fair Value of Assets	1,267	27,901
Derivative Liability, Fair Value, Gross Liability	22,120	31,223
Designated As Hedging Instrument \| Foreign Currency Forwards
Derivative [Line Items]
Notional Amount	150,211	0
Carrying Value / Fair Value of Assets	0	0
Derivative Liability, Fair Value, Gross Liability	4,158	0
Designated As Hedging Instrument \| Currency Swap [Member]
Derivative [Line Items]
Notional Amount	896,873	928,505
Carrying Value / Fair Value of Assets	86,318	104,359
Derivative Liability, Fair Value, Gross Liability	$ 7,176	$ 734

Derivative Instruments (Derivative Instruments Offsetting Balance Sheet) (Details) - USD ($)
$ in Thousands

Jun. 30, 2017

Dec. 31, 2016

Derivative Assets [Abstract]

Derivative Fair Value Of Free Standing Derivative Asset

$ 220,246

$ 283,094

Gross Amounts Offset in the Balance Sheet

(28,849)

(27,028)

Net Amounts Presented in the Balance Sheet

191,397

256,066

Derivative Asset, Fair Value of Collateral

^[1]

(17,847)

(16,913)

Gross Amounts Not Offset in the Balance Sheet - Cash Collateral Pledged/Received

^[2]

(183,179)

(254,498)

Net Amount

(9,629)

(15,345)

Derivative Liabilities [Abstract]

Derivative Fair Value Of Free Standing Derivative Liability

40,674

48,571

Gross Amounts Offset in the Balance Sheet

(28,849)

(27,028)

Net Amounts Presented in the Balance Sheet

11,825

21,543

Gross Amounts Not Offset in the Balance Sheet - Financial Instruments

^[3]

(59,540)

(95,863)

Gross Amounts Not Offset in the Balance Sheet - Cash Collateral Pledged/Received

^[4]

(7,461)

(1,441)

Net Amount

$ (55,176)

$ (75,761)

[1]	Consists of U.S. Treasury and government securities.
[2]	Included in cash and cash equivalents, with obligation to return cash collateral recorded in other liabilities.
[3]	Included in available-for-sale securities, primarily consists of U.S. Treasury and government agency securities.
[4]	Consists of receivable from counterparty, included in other assets.

Derivative Instruments Derivative Instruments (Fair Value Hedge Attributable to Foreign Currency) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Change in Unrealized Gain (Loss) on Fair Value Hedging Instruments	$ 905	$ (3,755)	$ 7,441	$ 2,112
Change in Unrealized Gain (Loss) on Hedged Item in Fair Value Hedge	(905)	3,755	(7,441)	(2,112)
Gain (Loss) on Fair Value Hedge Ineffectiveness, Net	$ 0	$ 0	$ 0	$ 0

Derivative Instruments (Current Period Cash Flow Hedges in AOCI (loss) before Taxes) (Details) - Cash Flow Hedging [Member] - USD ($) $ in Thousands	3 Months Ended				6 Months Ended		12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Mar. 31, 2016	Mar. 31, 2015	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	$ (1,131)	$ 41,192	$ (7,690)	$ 21,794	$ (1,131)	$ 41,192	$ 2,496	$ 29,397
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net	(6,417)	(20,350)			4,016	(11,799)
Investment Income [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	(183)	(58)			(430)	(165)
Gain (Loss) on Investments [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	$ 41	$ 1,010			$ 41	$ 169

Derivative Instruments (Cash Flow Hedges) (Details) - Cash Flow Hedging [Member] - USD ($) $ in Thousands	3 Months Ended				6 Months Ended		12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Mar. 31, 2016	Mar. 31, 2015	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net	$ (6,417)	$ (20,350)			$ 4,016	$ (11,799)
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	1,131	(41,192)	$ 7,690	$ (21,794)	1,131	(41,192)	$ (2,496)	$ (29,397)
Gain (Loss) on Investments [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	(41)	(1,010)			(41)	(169)
Investment Income [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	183	58			430	165
Interest Rate Swap [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net	(7,643)	(17,464)			(5,427)	(12,335)
Interest Rate Swap [Member] \| Gain (Loss) on Investments [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	0	0			0	0
Interest Rate Swap [Member] \| Investment Income [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	0	0			0	0
Currency Swap [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net	1,226	(2,886)			9,443	536
Currency Swap [Member] \| Gain (Loss) on Investments [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	0	0			0	0
Currency Swap [Member] \| Investment Income [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	132	93			329	253
Forward Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net	0	0			0	0
Forward Contracts [Member] \| Gain (Loss) on Investments [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	(41)	(1,010)			(41)	(169)
Forward Contracts [Member] \| Investment Income [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	$ 51	$ (35)			$ 101	$ (88)

Derivative Instruments (Hedges of Net Investments in Foreign Operations) (Details) - USD ($)
$ in Thousands

3 Months Ended

6 Months Ended

Jun. 30, 2017

Jun. 30, 2016

Jun. 30, 2017

Jun. 30, 2016

Derivatives, Fair Value [Line Items]

Net Derivatives Gains (Losses) Deferred in AOCI

^[1],[2]

$ (13,761)

$ 302

$ (21,367)

$ (31,493)

Currency Swap [Member]

Derivatives, Fair Value [Line Items]

Net Derivatives Gains (Losses) Deferred in AOCI

^[1],[2]

(17,919)

302

(25,525)

(31,493)

Foreign Exchange Forward [Member]

Derivatives, Fair Value [Line Items]

Net Derivatives Gains (Losses) Deferred in AOCI

^[1],[2]

$ 4,158

$ 0

$ 4,158

$ 0

[1]	There was no ineffectiveness recognized for the Company’s hedges of net investments in foreign operations.
[2]	There were no sales or substantial liquidations of net investments in foreign operations that would have required the reclassification of gains or losses from accumulated other comprehensive income (loss) into investment income during the periods presented.

Derivative Instruments (Non Hedging Derivatives and Embedded Derivatives Effect on Income Statement) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Income Affect Of Non Hedging Derivatives [Line Items]
Total investment related gains (losses), net	$ 56,295	$ 118,264	$ 116,818	$ (2,622)
Policy acquisition costs and other insurance expenses	(319,832)	(405,681)	(699,221)	(639,444)
Interest credited	(115,285)	(95,849)	(222,969)	(183,754)
Other revenues	73,992	66,193	142,149	125,376
Gain Loss On Investments Related To Change In Fair Value Of Derivatives Excluding Embedded Derivatives	3,684	40,733	(19,164)	94,835
Not Designated as Hedging Instrument
Income Affect Of Non Hedging Derivatives [Line Items]
Total investment related gains (losses), net	13,197	87,543	65,012	(12,151)
Not Designated as Hedging Instrument \| Interest Rate Swap [Member]
Income Affect Of Non Hedging Derivatives [Line Items]
Total investment related gains (losses), net	14,289	41,500	11,677	104,027
Not Designated as Hedging Instrument \| Financial Futures
Income Affect Of Non Hedging Derivatives [Line Items]
Total investment related gains (losses), net	(6,442)	(7,557)	(19,217)	(18,608)
Not Designated as Hedging Instrument \| Foreign Currency Forwards
Income Affect Of Non Hedging Derivatives [Line Items]
Total investment related gains (losses), net	(351)	3,577	553	6,077
Not Designated as Hedging Instrument \| Consumer Price Index Swap [Member]
Income Affect Of Non Hedging Derivatives [Line Items]
Total investment related gains (losses), net	(4)	(520)	(9)	(700)
Not Designated as Hedging Instrument \| Credit Default Swaps
Income Affect Of Non Hedging Derivatives [Line Items]
Total investment related gains (losses), net	3,879	3,518	11,237	6,864
Not Designated as Hedging Instrument \| Equity Options
Income Affect Of Non Hedging Derivatives [Line Items]
Total investment related gains (losses), net	(9,273)	(3,225)	(26,462)	(5,928)
Not Designated as Hedging Instrument \| Longevity Swap [Member]
Income Affect Of Non Hedging Derivatives [Line Items]
Other revenues	1,981	2,394	3,847	2,481
Not Designated as Hedging Instrument \| Mortality Swap [Member]
Income Affect Of Non Hedging Derivatives [Line Items]
Other revenues	(395)	1,046	(790)	622
Not Designated as Hedging Instrument \| Embedded Derivatives In Modified Coinsurance Or Funds Withheld Arrangements
Income Affect Of Non Hedging Derivatives [Line Items]
Total investment related gains (losses), net	15,108	76,966	83,810	(15,283)
Not Designated as Hedging Instrument \| Embedded Derivatives In Indexed Annuity [Member]
Income Affect Of Non Hedging Derivatives [Line Items]
Interest credited	(5,955)	(2,019)	(22,357)	(626)
Not Designated as Hedging Instrument \| Embedded Derivatives In Variable Annuity [Member]
Income Affect Of Non Hedging Derivatives [Line Items]
Total investment related gains (losses), net	$ 360	$ (28,137)	$ 22,723	$ (91,077)

Derivative Instruments (Exposure from Credit Derivatives by Rating of the Underlying Credits) (Details) - USD ($)
$ in Thousands

6 Months Ended

12 Months Ended

Jun. 30, 2017

Dec. 31, 2016

Maximum Potential Future Exposure on Credit Risk Derivatives

^[1]

$ 6,291

$ 8,970

^[1],[2]

$ 941,700

$ 922,700

Standard Poors AAA To A Minus Ratings [Member] | Single Name Credit Default Swaps [Member]

^[1],[3]

3 years 9 months

4 years 3 months

Maximum Potential Future Exposure on Credit Risk Derivatives

^[1]

$ 2,857

$ 1,726

^[1],[2]

$ 155,500

$ 150,500

Standard Poors AAA To A Minus Ratings [Member] | Credit Default Subtotal [Member]

^[1],[3]

3 years 6 months

3 years 10 months

Maximum Potential Future Exposure on Credit Risk Derivatives

^[1]

$ 2,857

$ 1,726

^[1],[2]

$ 155,500

$ 150,500

Standard Poors BBB Rating [Member] | Single Name Credit Default Swaps [Member]

^[1],[3]

3 years 6 months

3 years 10 months

Maximum Potential Future Exposure on Credit Risk Derivatives

^[1]

$ 3,351

$ 1,426

^[1],[2]

$ 365,200

$ 347,200

Standard Poors BBB Rating [Member] | Credit Default Swaps Referencing Indices [Member]

^[1],[3]

3 years 3 months

3 years 8 months

Maximum Potential Future Exposure on Credit Risk Derivatives

^[1]

$ 82

$ 6,295

^[1],[2]

$ 416,000

Standard Poors BBB Rating [Member] | Credit Default Subtotal [Member]

^[1],[3]

4 years 6 months

5 years

Maximum Potential Future Exposure on Credit Risk Derivatives

^[1]

$ 3,433

$ 7,721

^[1],[2]

$ 781,200

$ 763,200

Standard & Poor's, BB Rating [Member]

^[1],[3]

3 years 11 months

4 years 5 months

Standard & Poor's, BB Rating [Member] | Single Name Credit Default Swaps [Member]

^[1],[2]

2 years

3 years 6 months

Maximum Potential Future Exposure on Credit Risk Derivatives

^[1]

$ 1

$ (477)

^[1],[2]

5,000

9,000

Standard & Poor's, BB Rating [Member] | Credit Default Subtotal [Member]

Maximum Potential Future Exposure on Credit Risk Derivatives

^[1]

(477)

^[1],[2]

$ 5,000

$ 9,000

[1]	The rating agency designations are based on ratings from Standard and Poor’s (“S&P”).
[2]	Assumes the value of the referenced credit obligations is zero.
[3]	The weighted average years to maturity of the credit default swaps is calculated based on weighted average notional amounts.

Derivative Instruments (Changes in Estimated Fair Value Related to Embedded Derivatives) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Changes In Estimated Fair Value Related To Embedded Derivatives [Line Items]
Gain Loss On Investments Related To Change In Fair Value Of Derivatives Excluding Embedded Derivatives	$ 3,684	$ 40,733	$ (19,164)	$ 94,835
Embedded Derivatives In Modified Coinsurance Or Funds Withheld Arrangements
Changes In Estimated Fair Value Related To Embedded Derivatives [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	15,108	76,966	83,810	(15,283)
Derivative, Gain (Loss) on Derivative, Net	2,941	18,807	28,785	(7,970)
Embedded Derivatives In Equity Indexed Annuities [Member]
Changes In Estimated Fair Value Related To Embedded Derivatives [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	(5,955)	(2,019)	(22,357)	(626)
Derivative, Gain (Loss) on Derivative, Net	(6,925)	(7,816)	(28,322)	3,418
Embedded Derivatives In Variable Annuity Contracts [Member]
Changes In Estimated Fair Value Related To Embedded Derivatives [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	360	(28,137)	22,723	(91,077)
Derivative, Gain (Loss) on Derivative, Net	$ 3,023	$ (40,167)	$ 31,859	$ (66,010)

Derivative Instruments (Credit Exposure of Derivative Contracts) (Details) - USD ($)
$ in Thousands

Jun. 30, 2017

Dec. 31, 2016

Credit Exposure of Derivative Contracts

Estimated fair value of derivatives in net asset (liability) position

$ 181,124

$ 236,985

Derivative, Collateral, Right to Reclaim Cash

^[1]

7,461

1,441

Derivative Liability, Fair Value of Collateral

^[2]

59,540

95,863

Derivative, Collateral, Obligation to Return Cash

^[3]

(183,179)

(254,498)

Derivative Asset, Fair Value of Collateral

^[4]

(17,847)

(16,913)

Deposits with Clearing Organizations and Others, Securities

^[2]

(58,526)

(73,571)

Net credit exposure

(11,427)

(10,693)

Margin account related to exchange-traded futures

^[5]

$ 8,530

$ 9,687

[1]	Consists of receivable from counterparty, included in other assets.
[2]	Included in available-for-sale securities, primarily consists of U.S. Treasury and government agency securities.
[3]	Included in cash and cash equivalents, with obligation to return cash collateral recorded in other liabilities.
[4]	Consists of U.S. Treasury and government securities.
[5]	Included in other assets.

Derivative Instruments (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net	$ 0.5		$ 0.5
Derivative Instruments, Gain Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing		$ 2.4		$ 7.0
Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net	(0.1)		(0.1)
Cumulative Foreign Currency Translation Gain Loss Recorded In Accumulated Other Comprehensive Income Loss For Terminated Net Investment In Foreign Operations Hedges	$ 140.3		$ 140.3		$ 161.6

Fair Value of Assets and Liabilities Fair Value of Assets and Liabilities (Quantitative Information for Level 3 Inputs) (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Percentage Of Fixed Maturity Securities Classified As Level 3	6.20%	6.90%
Corporate Debt Securities [Member] \| Market Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 158,309	$ 167,815
Corporate Debt Securities [Member] \| Weighted Average [Member] \| Market Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount for Lack of Marketability	1.00%	1.00%
Corporate Debt Securities [Member] \| Maximum [Member] \| Market Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount for Lack of Marketability	2.00%	2.00%
Corporate Debt Securities [Member] \| Minimum [Member] \| Market Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount for Lack of Marketability	0.00%	0.00%
Longevity Swap [Member] \| Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 33,349	$ 26,958
Longevity Swap [Member] \| Weighted Average [Member] \| Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Long-Duration Contracts, Assumptions by Product and Guarantee, Mortality Rate, Low End	2.00%	2.00%
Fair Value Inputs Mortality Improvement	3.00%	3.00%
Longevity Swap [Member] \| Maximum [Member] \| Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Long-Duration Contracts, Assumptions by Product and Guarantee, Mortality Rate, Low End	100.00%	100.00%
Fair Value Inputs Mortality Improvement	10.00%	10.00%
Longevity Swap [Member] \| Minimum [Member] \| Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Long-Duration Contracts, Assumptions by Product and Guarantee, Mortality Rate, Low End	0.00%	0.00%
Fair Value Inputs Mortality Improvement	(10.00%)	(10.00%)
US Government Agencies Debt Securities [Member] \| Weighted Average [Member] \| Market Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount for Lack of Marketability	1.00%	1.00%
US Government Agencies Debt Securities [Member] \| Maximum [Member] \| Market Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount for Lack of Marketability	1.00%	1.00%
US Government Agencies Debt Securities [Member] \| Minimum [Member] \| Market Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount for Lack of Marketability	0.00%	0.00%
US States and Political Subdivisions Debt Securities [Member] \| Market Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 4,607	$ 4,670
US States and Political Subdivisions Debt Securities [Member] \| Weighted Average [Member] \| Market Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount for Lack of Marketability	1.00%	1.00%
US States and Political Subdivisions Debt Securities [Member] \| Maximum [Member] \| Market Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount for Lack of Marketability	1.00%	1.00%
US States and Political Subdivisions Debt Securities [Member] \| Minimum [Member] \| Market Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount for Lack of Marketability	1.00%	1.00%
Embedded Derivatives In Indexed Annuity [Member] \| Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Financial and Nonfinancial Liabilities, Fair Value Disclosure	$ 812,718	$ 805,672
Embedded Derivatives In Indexed Annuity [Member] \| Weighted Average [Member] \| Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Long-Duration Contracts, Assumptions by Product and Guarantee, Lapse Rate, Low End	9.00%	8.00%
Long-Duration Contracts, Assumptions by Product and Guarantee, Mortality Rate, Low End	2.00%	2.00%
Fair Value Inputs Withdrawal	3.00%	3.00%
Fair Value Inputs Option Budget Projection	2.00%	2.00%
Embedded Derivatives In Indexed Annuity [Member] \| Maximum [Member] \| Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Long-Duration Contracts, Assumptions by Product and Guarantee, Lapse Rate, Low End	35.00%	35.00%
Long-Duration Contracts, Assumptions by Product and Guarantee, Mortality Rate, Low End	100.00%	100.00%
Fair Value Inputs Withdrawal	5.00%	5.00%
Fair Value Inputs Option Budget Projection	4.00%	4.00%
Embedded Derivatives In Indexed Annuity [Member] \| Minimum [Member] \| Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Long-Duration Contracts, Assumptions by Product and Guarantee, Lapse Rate, Low End	0.00%	0.00%
Long-Duration Contracts, Assumptions by Product and Guarantee, Mortality Rate, Low End	0.00%	0.00%
Fair Value Inputs Withdrawal	0.00%	0.00%
Fair Value Inputs Option Budget Projection	2.00%	2.00%
Embedded Derivatives In Variable Annuity [Member] \| Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Financial and Nonfinancial Liabilities, Fair Value Disclosure	$ 161,913	$ 184,636
Embedded Derivatives In Variable Annuity [Member] \| Weighted Average [Member] \| Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Long-Duration Contracts, Assumptions by Product and Guarantee, Lapse Rate, Low End	6.00%	6.00%
Long-Duration Contracts, Assumptions by Product and Guarantee, Mortality Rate, Low End	2.00%	2.00%
Fair Value Inputs Withdrawal	3.00%	3.00%
Embedded Derivatives In Variable Annuity [Member] \| Maximum [Member] \| Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Long-Duration Contracts, Assumptions by Product and Guarantee, Lapse Rate, Low End	25.00%	25.00%
Long-Duration Contracts, Assumptions by Product and Guarantee, Mortality Rate, Low End	100.00%	100.00%
Fair Value Inputs Withdrawal	7.00%	7.00%
Embedded Derivatives In Variable Annuity [Member] \| Minimum [Member] \| Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Long-Duration Contracts, Assumptions by Product and Guarantee, Lapse Rate, Low End	0.00%	0.00%
Long-Duration Contracts, Assumptions by Product and Guarantee, Mortality Rate, Low End	0.00%	0.00%
Fair Value Inputs Withdrawal	0.00%	0.00%
Embedded Derivatives In Variable Annuity Contracts [Member] \| Weighted Average [Member] \| Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Entity Credit Risk	1.00%	1.00%
Fair Value Assumptions, Expected Volatility Rate	9.00%	14.00%
Embedded Derivatives In Variable Annuity Contracts [Member] \| Maximum [Member] \| Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Entity Credit Risk	5.00%	5.00%
Fair Value Assumptions, Expected Volatility Rate	27.00%	27.00%
Embedded Derivatives In Variable Annuity Contracts [Member] \| Minimum [Member] \| Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Entity Credit Risk	0.00%	0.00%
Fair Value Assumptions, Expected Volatility Rate	0.00%	0.00%
Mortality Swap [Member] \| Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Financial and Nonfinancial Liabilities, Fair Value Disclosure	$ 1,552	$ 2,462
Mortality Swap [Member] \| Weighted Average [Member] \| Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Long-Duration Contracts, Assumptions by Product and Guarantee, Mortality Rate, Low End	1.00%	1.00%
Mortality Swap [Member] \| Maximum [Member] \| Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Long-Duration Contracts, Assumptions by Product and Guarantee, Mortality Rate, Low End	100.00%	100.00%
Mortality Swap [Member] \| Minimum [Member] \| Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Long-Duration Contracts, Assumptions by Product and Guarantee, Mortality Rate, Low End	0.00%	0.00%
Total Return Swap [Member] \| Funds Withheld At Interest Embedded Derivatives [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 61,281	$ (22,529)
Total Return Swap [Member] \| Funds Withheld At Interest Embedded Derivatives [Member] \| Weighted Average [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Long-Duration Contracts, Assumptions by Product and Guarantee, Lapse Rate, Low End	9.00%	8.00%
Long-Duration Contracts, Assumptions by Product and Guarantee, Mortality Rate, Low End	2.00%	2.00%
Fair Value Inputs Withdrawal	3.00%	3.00%
Fair Value Inputs, Entity Credit Risk	1.00%	1.00%
Fair Value Inputs Credit Rate	2.00%	2.00%
Total Return Swap [Member] \| Funds Withheld At Interest Embedded Derivatives [Member] \| Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Long-Duration Contracts, Assumptions by Product and Guarantee, Lapse Rate, Low End	35.00%	35.00%
Long-Duration Contracts, Assumptions by Product and Guarantee, Mortality Rate, Low End	100.00%	100.00%
Fair Value Inputs Withdrawal	5.00%	5.00%
Fair Value Inputs, Entity Credit Risk	5.00%	5.00%
Fair Value Inputs Credit Rate	4.00%	4.00%
Total Return Swap [Member] \| Funds Withheld At Interest Embedded Derivatives [Member] \| Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Long-Duration Contracts, Assumptions by Product and Guarantee, Lapse Rate, Low End	0.00%	0.00%
Long-Duration Contracts, Assumptions by Product and Guarantee, Mortality Rate, Low End	0.00%	0.00%
Fair Value Inputs Withdrawal	0.00%	0.00%
Fair Value Inputs, Entity Credit Risk	0.00%	0.00%
Fair Value Inputs Credit Rate	2.00%	2.00%
Total Return Swap [Member] \| US Government Agencies Debt Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 23,567	$ 24,488

Fair Value of Assets and Liabilities Fair Value of Assets and Liabilities (Fair Value Assets Transfers between Level 1 and Level 2) (Details) - Corporate Debt Securities [Member] - Available-for-sale Securities [Member]	3 Months Ended	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Equity, Level 1 to Level 2 Transfers, Description	0	0
Fair Value, Equity, Level 2 to Level 1 Transfers, Description	49999	88674

Fair Value of Assets and Liabilities Fair Value of Assets and Liabilities (Fair Value Assets Measured on Non-Recurring Basis) (Details) - USD ($)
$ in Thousands

3 Months Ended

6 Months Ended

Jun. 30, 2017

Jun. 30, 2016

Jun. 30, 2017

Jun. 30, 2016

Mortgage Loans on Real Estate [Member]

Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]

Fair Value Assets Carrying Value Prior To Impairment

^[1]

$ 0

$ 6,993

Fair Value Assets Gains Losses

^[1]

$ 0

$ (400)

(702)

Partnership Interest [Member]

Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]

Fair Value Assets Carrying Value Prior To Impairment

^[2]

3,690

4,460

Fair Value Assets Gains Losses

^[2]

$ (6,308)

$ (112)

$ (6,308)

$ (2,039)

[1]	Estimated fair values for impaired mortgage loans are based on internal valuation models using unobservable inputs or, if the loans are in foreclosure or are otherwise determined to be collateral dependent, are based on external appraisals of the underlying collateral
[2]	The impaired limited partnership interests presented above were accounted for using the cost method. Impairments on these cost method investments were recognized at estimated fair value determined using the net asset values of the Company’s ownership interest as provided in the financial statements of the investees. The market for these investments has limited activity and price transparency.

Fair Value of Assets and Liabilities Fair Value of Assets and Liabilities (Financial Instruments where Carrying Amounts and Fair Values May Differ) (Details) - USD ($)
$ in Thousands

Jun. 30, 2017

Dec. 31, 2016

Jun. 30, 2016

Dec. 31, 2015

Assets

Mortgage loans on real estate (net of valuation allowances)

$ 4,104,487

$ 3,775,522

1,406,774

1,427,602

5,968,856

5,875,919

1,123,350

1,200,718

$ 1,034,329

$ 1,525,275

123,308

76,710

1,498,370

1,591,940

388,008

347,173

16,440,873

14,029,354

Long-term debt

2,788,494

3,088,635

Reported Value Measurement [Member]

823,108

840,700

Assets

Mortgage loans on real estate (net of valuation allowances)

4,104,487

3,775,522

1,406,774

1,427,602

^[1]

5,904,679

5,893,381

^[2]

822,834

862,117

^[2]

32,284

32,469

^[2]

567,448

477,132

388,008

347,173

^[1]

12,718,836

10,225,099

Long-term debt

2,788,494

3,088,635

Estimate of Fair Value Measurement [Member]

823,108

840,700

Assets

Mortgage loans on real estate (net of valuation allowances)

4,215,094

3,786,987

1,406,774

1,427,602

^[1]

6,258,874

6,193,166

^[2]

822,834

862,117

^[2]

32,284

32,469

^[2]

603,783

510,640

388,008

347,173

^[1]

12,711,801

10,234,544

3,029,601

3,186,173

Estimate of Fair Value Measurement [Member] | Net Asset Value [Member]

730,809

745,805

Assets

Mortgage loans on real estate (net of valuation allowances)

^[1]

^[2]

^[2]

^[2]

310,536

296,773

^[1]

Estimate of Fair Value Measurement [Member] | Fair Value Inputs Level 1 Member

Assets

Mortgage loans on real estate (net of valuation allowances)

^[1]

^[2]

822,834

862,117

^[2]

32,284

32,469

^[2]

27,815

26,294

^[1]

Estimate of Fair Value Measurement [Member] | Fair Value Inputs Level 2 Member

Assets

Mortgage loans on real estate (net of valuation allowances)

1,406,774

1,427,602

^[1]

^[2]

^[2]

^[2]

66,501

55,669

388,008

347,173

^[1]

Estimate of Fair Value Measurement [Member] | Fair Value Inputs Level 3 Member

Assets

Mortgage loans on real estate (net of valuation allowances)

4,215,094

3,786,987

^[1]

6,258,874

6,193,166

^[2]

^[2]

^[2]

198,931

131,904

^[1]

12,711,801

10,234,544

3,029,601

3,186,173