Investments (Fixed Maturities Securities Excluding Investments Classified as Trading) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 10,191,008	$ 16,177,891
Allowance for Credit Losses	2,226	686
Fair Value	10,577,150	18,584,685
Fixed maturities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	10,191,008	16,177,891
Gross Unrealized Gains	448,475	2,431,362
Gross Unrealized Losses	60,107	23,882
Allowance for Credit Losses	2,226	686
Fair Value	10,577,150	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	669,350	7,283,403
Gross Unrealized Gains	57,637	1,631,715
Gross Unrealized Losses	302	12,686
Allowance for Credit Losses	0	0
Fair Value	726,685	8,902,432
Fixed maturities \| US States and Political Subdivisions Debt Securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	283,487	258,135
Gross Unrealized Gains	14,845	19,512
Gross Unrealized Losses	883	156
Allowance for Credit Losses	0	0
Fair Value	297,449	277,491
Fixed maturities \| Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	164,023	153,009
Gross Unrealized Gains	14,207	24,378
Gross Unrealized Losses	813	4
Allowance for Credit Losses	0	0
Fair Value	177,417	177,383
Fixed maturities \| U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,843,576	3,112,420
Gross Unrealized Gains	180,349	339,348
Gross Unrealized Losses	16,855	1,361
Allowance for Credit Losses	0	0
Fair Value	3,007,070	3,450,407
Fixed maturities \| U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,037,889	1,736,035
Gross Unrealized Gains	73,950	155,943
Gross Unrealized Losses	6,759	4,706
Allowance for Credit Losses	2,226	209
Fair Value	2,102,854	1,887,063
Fixed maturities \| Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	744,178	513,204
Gross Unrealized Gains	17,155	29,029
Gross Unrealized Losses	3,725	408
Allowance for Credit Losses	0	0
Fair Value	757,608	541,825
Fixed maturities \| Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,311,604	1,338,936
Gross Unrealized Gains	43,078	158,227
Gross Unrealized Losses	23,911	2,851
Allowance for Credit Losses	0	477
Fair Value	1,330,771	1,493,835
Fixed maturities \| Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,266,654	984,318
Gross Unrealized Gains	7,321	9,870
Gross Unrealized Losses	311	1,605
Allowance for Credit Losses	0	0
Fair Value	1,273,664	992,583
Fixed maturities \| Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	793,065	728,522
Gross Unrealized Gains	36,371	57,522
Gross Unrealized Losses	6,519	102
Allowance for Credit Losses	0	0
Fair Value	822,917	785,942
Fixed maturities \| Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	77,182	69,909
Gross Unrealized Gains	3,562	5,818
Gross Unrealized Losses	29	3
Allowance for Credit Losses	0	0
Fair Value	$ 80,715	$ 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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