Investments (Fixed Maturities Securities Excluding Investments Classified as Trading) (Details) - USD ($) $ in Thousands	Jun. 30, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 25,557,874	$ 21,311,087
Allowance for Credit Losses	4,923	4,769
Fair Value	23,441,750	19,025,401
Fixed maturities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	25,557,874	21,311,087
Gross Unrealized Gains	104,891	72,582
Gross Unrealized Losses	2,216,092	2,353,499
Allowance for Credit Losses	4,923	4,769
Fair Value	23,441,750	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	392,585	354,348
Gross Unrealized Gains	258	300
Gross Unrealized Losses	68,582	72,856
Allowance for Credit Losses	0	0
Fair Value	324,261	281,792
Fixed maturities | Obligations of U.S. states and their political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	797,759	654,884
Gross Unrealized Gains	3,900	4,275
Gross Unrealized Losses	24,568	30,959
Allowance for Credit Losses	0	0
Fair Value	777,091	628,200
Fixed maturities | Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	341,979	330,967
Gross Unrealized Gains	1,229	1,140
Gross Unrealized Losses	54,900	58,640
Allowance for Credit Losses	0	5
Fair Value	288,308	273,462
Fixed maturities | U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	9,797,076	7,414,790
Gross Unrealized Gains	29,259	21,299
Gross Unrealized Losses	940,277	992,145
Allowance for Credit Losses	1,175	0
Fair Value	8,884,883	6,443,944
Fixed maturities | U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,659,960	4,140,734
Gross Unrealized Gains	12,365	13,071
Gross Unrealized Losses	340,940	335,205
Allowance for Credit Losses	3,480	1,871
Fair Value	4,327,905	3,816,729
Fixed maturities | Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,873,238	1,539,172
Gross Unrealized Gains	2,455	2,455
Gross Unrealized Losses	155,979	163,384
Allowance for Credit Losses	262	21
Fair Value	1,719,452	1,378,222
Fixed maturities | Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,521,673	4,338,585
Gross Unrealized Gains	38,274	19,761
Gross Unrealized Losses	520,224	589,153
Allowance for Credit Losses	0	2,863
Fair Value	4,039,723	3,766,330
Fixed maturities | Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,903,744	1,467,955
Gross Unrealized Gains	15,309	6,976
Gross Unrealized Losses	21,487	32,577
Allowance for Credit Losses	0	0
Fair Value	1,897,566	1,442,354
Fixed maturities | Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	858,875	727,159
Gross Unrealized Gains	49	94
Gross Unrealized Losses	78,819	69,101
Allowance for Credit Losses	0	0
Fair Value	780,105	658,152
Fixed maturities | Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	410,985	342,493
Gross Unrealized Gains	1,793	3,211
Gross Unrealized Losses	10,316	9,479
Allowance for Credit Losses	6	9
Fair Value	$ 402,456	$ 336,216