Investments (Fixed Maturities Securities Excluding Investments Classified as Trading) (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 8,556,236	$ 16,177,891
Allowance for Credit Losses	1,086	686
Fair Value	8,770,966	18,584,685
Fixed maturities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	8,556,236	16,177,891
Gross Unrealized Gains	301,061	2,431,362
Gross Unrealized Losses	85,245	23,882
Allowance for Credit Losses	1,086	686
Fair Value	8,770,966	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	654,389	7,283,403
Gross Unrealized Gains	73,618	1,631,715
Gross Unrealized Losses	1,182	12,686
Allowance for Credit Losses	0	0
Fair Value	726,825	8,902,432
Fixed maturities \| US States and Political Subdivisions Debt Securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	280,875	258,135
Gross Unrealized Gains	14,800	19,512
Gross Unrealized Losses	15	156
Allowance for Credit Losses	0	0
Fair Value	295,660	277,491
Fixed maturities \| Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	106,240	153,009
Gross Unrealized Gains	8,848	24,378
Gross Unrealized Losses	722	4
Allowance for Credit Losses	0	0
Fair Value	114,366	177,383
Fixed maturities \| U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,344,782	3,112,420
Gross Unrealized Gains	132,929	339,348
Gross Unrealized Losses	17,743	1,361
Allowance for Credit Losses	0	0
Fair Value	2,459,968	3,450,407
Fixed maturities \| U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,635,200	1,736,035
Gross Unrealized Gains	24,316	155,943
Gross Unrealized Losses	13,858	4,706
Allowance for Credit Losses	1,086	209
Fair Value	1,644,572	1,887,063
Fixed maturities \| Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	792,832	513,204
Gross Unrealized Gains	11,317	29,029
Gross Unrealized Losses	9,126	408
Allowance for Credit Losses	0	0
Fair Value	795,023	541,825
Fixed maturities \| Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	920,062	1,338,936
Gross Unrealized Gains	5,361	158,227
Gross Unrealized Losses	33,706	2,851
Allowance for Credit Losses	0	477
Fair Value	891,717	1,493,835
Fixed maturities \| Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,057,995	984,318
Gross Unrealized Gains	5,120	9,870
Gross Unrealized Losses	757	1,605
Allowance for Credit Losses	0	0
Fair Value	1,062,358	992,583
Fixed maturities \| Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	692,232	728,522
Gross Unrealized Gains	22,149	57,522
Gross Unrealized Losses	8,061	102
Allowance for Credit Losses	0	0
Fair Value	706,320	785,942
Fixed maturities \| Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	71,629	69,909
Gross Unrealized Gains	2,603	5,818
Gross Unrealized Losses	75	3
Allowance for Credit Losses	0	0
Fair Value	$ 74,157	$ 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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