Investments (Fixed Maturities Securities Excluding Investments Classified as Trading) (Details) - USD ($) $ in Thousands	Mar. 31, 2025	Dec. 31, 2024	Mar. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 40,807,986	$ 36,980,933
Gross Unrealized Gains	331,892	208,233
Gross Unrealized Losses	1,930,978	2,162,592
Allowance for Credit Losses	42,828	40,414	$ 1,650	$ 2,008
Fair Value	39,166,072	34,986,160
U.S. Treasury securities and obligations of U.S. government authorities and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,150,479	1,199,628
Gross Unrealized Gains	19,881	8,357
Gross Unrealized Losses	89,700	108,744
Allowance for Credit Losses	0	0	0	0
Fair Value	1,080,660	1,099,241
Obligations of U.S. states and their political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	551,375	570,253
Gross Unrealized Gains	1,254	1,156
Gross Unrealized Losses	35,445	30,343
Allowance for Credit Losses	0	0
Fair Value	517,184	541,066
Foreign government securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	439,135	362,154
Gross Unrealized Gains	3,108	646
Gross Unrealized Losses	49,006	52,466
Allowance for Credit Losses	0	0	0	0
Fair Value	393,237	310,334
U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	16,328,435	14,134,828
Gross Unrealized Gains	124,879	60,917
Gross Unrealized Losses	892,473	957,316
Allowance for Credit Losses	19	1
Fair Value	15,560,822	13,238,428
U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6,538,902	6,030,898
Gross Unrealized Gains	29,688	35,828
Gross Unrealized Losses	250,640	301,451
Allowance for Credit Losses	9,164	11,178
Fair Value	6,308,786	5,754,097
Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,439,517	3,804,503
Gross Unrealized Gains	46,832	21,136
Gross Unrealized Losses	113,026	126,767
Allowance for Credit Losses	21	21
Fair Value	4,373,302	3,698,851
Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6,180,964	5,838,939
Gross Unrealized Gains	70,080	43,334
Gross Unrealized Losses	437,381	511,426
Allowance for Credit Losses	33,624	29,214
Fair Value	5,780,039	5,341,633
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,801,789	3,728,073
Gross Unrealized Gains	25,349	31,431
Gross Unrealized Losses	10,782	8,841
Allowance for Credit Losses	0	0	0	1
Fair Value	3,816,356	3,750,663
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,018,053	944,652
Gross Unrealized Gains	8,052	4,567
Gross Unrealized Losses	44,565	53,444
Allowance for Credit Losses	0	0	0	0
Fair Value	981,540	895,775
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	359,337	367,005
Gross Unrealized Gains	2,769	861
Gross Unrealized Losses	7,960	11,794
Allowance for Credit Losses	0	0	$ 0	$ 7
Fair Value	$ 354,146	$ 356,072