Investments (Fixed Maturities Securities Excluding Investments Classified as Trading) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 11,178,085	$ 16,177,891
Allowance for Credit Losses	735	686
Fair Value	11,525,964	18,584,685
Fixed maturities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	11,178,085	16,177,891
Gross Unrealized Gains	484,903	2,431,362
Gross Unrealized Losses	136,289	23,882
Allowance for Credit Losses	735	686
Fair Value	11,525,964	18,584,685
Fixed maturities \| U.S. Treasury securities and obligations of U.S. government authorities and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	914,937	7,283,403
Gross Unrealized Gains	30,951	1,631,715
Gross Unrealized Losses	10,334	12,686
Allowance for Credit Losses	0	0
Fair Value	935,554	8,902,432
Fixed maturities \| US States and Political Subdivisions Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	302,372	258,135
Gross Unrealized Gains	12,297	19,512
Gross Unrealized Losses	2,340	156
Allowance for Credit Losses	0	0
Fair Value	312,329	277,491
Fixed maturities \| Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	172,078	153,009
Gross Unrealized Gains	16,143	24,378
Gross Unrealized Losses	896	4
Allowance for Credit Losses	0	0
Fair Value	187,325	177,383
Fixed maturities \| U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,785,253	3,112,420
Gross Unrealized Gains	167,327	339,348
Gross Unrealized Losses	71,551	1,361
Allowance for Credit Losses	0	0
Fair Value	3,881,029	3,450,407
Fixed maturities \| U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,767,324	1,736,035
Gross Unrealized Gains	88,125	155,943
Gross Unrealized Losses	14,741	4,706
Allowance for Credit Losses	720	209
Fair Value	1,839,988	1,887,063
Fixed maturities \| Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	902,623	513,204
Gross Unrealized Gains	18,051	29,029
Gross Unrealized Losses	14,049	408
Allowance for Credit Losses	0	0
Fair Value	906,625	541,825
Fixed maturities \| Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,457,455	1,338,936
Gross Unrealized Gains	102,363	158,227
Gross Unrealized Losses	11,411	2,851
Allowance for Credit Losses	15	477
Fair Value	1,548,392	1,493,835
Fixed maturities \| Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	975,298	984,318
Gross Unrealized Gains	9,902	9,870
Gross Unrealized Losses	212	1,605
Allowance for Credit Losses	0	0
Fair Value	984,988	992,583
Fixed maturities \| Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	835,555	728,522
Gross Unrealized Gains	35,109	57,522
Gross Unrealized Losses	10,722	102
Allowance for Credit Losses	0	0
Fair Value	859,942	785,942
Fixed maturities \| Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	65,190	69,909
Gross Unrealized Gains	4,635	5,818
Gross Unrealized Losses	33	3
Allowance for Credit Losses	0	0
Fair Value	$ 69,792	$ 75,724

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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