Investments (Fixed Maturities Securities Excluding Investments Classified as Trading) (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 26,682,558	$ 21,311,087
Allowance for Credit Losses	4,315	4,769
Fair Value	23,798,355	19,025,401
Fixed maturities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	26,682,558	21,311,087
Gross Unrealized Gains	51,338	72,582
Gross Unrealized Losses	2,931,226	2,353,499
Allowance for Credit Losses	4,315	4,769
Fair Value	23,798,355	19,025,401
Fixed maturities | U.S. Treasury securities and obligations of U.S. government authorities and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	392,452	354,348
Gross Unrealized Gains	119	300
Gross Unrealized Losses	99,511	72,856
Allowance for Credit Losses	0	0
Fair Value	293,060	281,792
Fixed maturities | Obligations of U.S. states and their political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	794,103	654,884
Gross Unrealized Gains	562	4,275
Gross Unrealized Losses	62,748	30,959
Allowance for Credit Losses	0	0
Fair Value	731,917	628,200
Fixed maturities | Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	341,894	330,967
Gross Unrealized Gains	628	1,140
Gross Unrealized Losses	75,489	58,640
Allowance for Credit Losses	0	5
Fair Value	267,033	273,462
Fixed maturities | U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	10,141,650	7,414,790
Gross Unrealized Gains	9,571	21,299
Gross Unrealized Losses	1,313,950	992,145
Allowance for Credit Losses	1,097	0
Fair Value	8,836,174	6,443,944
Fixed maturities | U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,958,820	4,140,734
Gross Unrealized Gains	3,596	13,071
Gross Unrealized Losses	427,455	335,205
Allowance for Credit Losses	2,946	1,871
Fair Value	4,532,015	3,816,729
Fixed maturities | Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,883,846	1,539,172
Gross Unrealized Gains	1,134	2,455
Gross Unrealized Losses	190,248	163,384
Allowance for Credit Losses	264	21
Fair Value	1,694,468	1,378,222
Fixed maturities | Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,802,652	4,338,585
Gross Unrealized Gains	19,942	19,761
Gross Unrealized Losses	622,422	589,153
Allowance for Credit Losses	0	2,863
Fair Value	4,200,172	3,766,330
Fixed maturities | Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,037,295	1,467,955
Gross Unrealized Gains	15,615	6,976
Gross Unrealized Losses	23,202	32,577
Allowance for Credit Losses	0	0
Fair Value	2,029,708	1,442,354
Fixed maturities | Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	926,653	727,159
Gross Unrealized Gains	32	94
Gross Unrealized Losses	93,898	69,101
Allowance for Credit Losses	0	0
Fair Value	832,787	658,152
Fixed maturities | Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	403,193	342,493
Gross Unrealized Gains	139	3,211
Gross Unrealized Losses	22,303	9,479
Allowance for Credit Losses	8	9
Fair Value	$ 381,021	$ 336,216