Investments (Fixed Maturities Securities Excluding Investments Classified as Trading) (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 8,888,296	$ 8,556,236
Allowance for Credit Losses	1,438	1,086
Fair Value	8,522,040	8,770,966
Fixed maturities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	8,888,296	8,556,236
Gross Unrealized Gains	48,577	301,061
Gross Unrealized Losses	413,395	85,245
Allowance for Credit Losses	1,438	1,086
Fair Value	8,522,040	8,770,966
Fixed maturities \| U.S. Treasury securities and obligations of U.S. government authorities and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	890,970	654,389
Gross Unrealized Gains	879	73,618
Gross Unrealized Losses	11,532	1,182
Allowance for Credit Losses	0	0
Fair Value	880,317	726,825
Fixed maturities \| US States and Political Subdivisions Debt Securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	260,080	280,875
Gross Unrealized Gains	2,389	14,800
Gross Unrealized Losses	5,863	15
Allowance for Credit Losses	0	0
Fair Value	256,606	295,660
Fixed maturities \| Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	96,605	106,240
Gross Unrealized Gains	4,093	8,848
Gross Unrealized Losses	2,197	722
Allowance for Credit Losses	0	0
Fair Value	98,501	114,366
Fixed maturities \| U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,456,029	2,344,782
Gross Unrealized Gains	30,784	132,929
Gross Unrealized Losses	110,327	17,743
Allowance for Credit Losses	0	0
Fair Value	2,376,486	2,459,968
Fixed maturities \| U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,618,621	1,635,200
Gross Unrealized Gains	1,287	24,316
Gross Unrealized Losses	100,472	13,858
Allowance for Credit Losses	1,438	1,086
Fair Value	1,517,998	1,644,572
Fixed maturities \| Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	861,488	792,832
Gross Unrealized Gains	2,448	11,317
Gross Unrealized Losses	51,253	9,126
Allowance for Credit Losses	0	0
Fair Value	812,683	795,023
Fixed maturities \| Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	899,035	920,062
Gross Unrealized Gains	596	5,361
Gross Unrealized Losses	92,090	33,706
Allowance for Credit Losses	0	0
Fair Value	807,541	891,717
Fixed maturities \| Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,028,440	1,057,995
Gross Unrealized Gains	2,264	5,120
Gross Unrealized Losses	8,025	757
Allowance for Credit Losses	0	0
Fair Value	1,022,679	1,062,358
Fixed maturities \| Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	719,001	692,232
Gross Unrealized Gains	2,823	22,149
Gross Unrealized Losses	31,113	8,061
Allowance for Credit Losses	0	0
Fair Value	690,711	706,320
Fixed maturities \| Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	58,027	71,629
Gross Unrealized Gains	1,014	2,603
Gross Unrealized Losses	523	75
Allowance for Credit Losses	0	0
Fair Value	$ 58,518	$ 74,157

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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