Investments (Fixed Maturities Securities Excluding Investments Classified as Trading) (Details) - USD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 36,980,933	$ 27,538,066
Gross Unrealized Gains	208,233	335,759
Gross Unrealized Losses	2,162,592	1,740,037
Allowance for credit losses	40,414	2,008	$ 4,769
Fair Value	34,986,160	26,131,780
U.S. Treasury securities and obligations of U.S. government authorities and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,199,628	1,009,937
Gross Unrealized Gains	8,357	38,858
Gross Unrealized Losses	108,744	73,508
Allowance for credit losses	0	0	0
Fair Value	1,099,241	975,287
Obligations of U.S. states and their political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	570,253	789,856
Gross Unrealized Gains	1,156	5,288
Gross Unrealized Losses	30,343	18,517
Allowance for credit losses	0	0
Fair Value	541,066	776,627
Foreign government securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	362,154	330,830
Gross Unrealized Gains	646	1,840
Gross Unrealized Losses	52,466	50,684
Allowance for credit losses	0	0	5
Fair Value	310,334	281,986
U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	14,134,828	10,159,089
Gross Unrealized Gains	60,917	98,047
Gross Unrealized Losses	957,316	760,274
Allowance for credit losses	1	950
Fair Value	13,238,428	9,495,912
U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6,030,898	5,207,699
Gross Unrealized Gains	35,828	37,435
Gross Unrealized Losses	301,451	254,828
Allowance for credit losses	11,178	812
Fair Value	5,754,097	4,989,494
Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,804,503	1,809,347
Gross Unrealized Gains	21,136	12,658
Gross Unrealized Losses	126,767	115,673
Allowance for credit losses	21	238
Fair Value	3,698,851	1,706,094
Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,838,939	4,902,391
Gross Unrealized Gains	43,334	109,806
Gross Unrealized Losses	511,426	381,215
Allowance for credit losses	29,214	0
Fair Value	5,341,633	4,630,982
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,728,073	2,016,028
Gross Unrealized Gains	31,431	23,035
Gross Unrealized Losses	8,841	11,512
Allowance for credit losses	0	1	0
Fair Value	3,750,663	2,027,550
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	944,652	913,347
Gross Unrealized Gains	4,567	4,776
Gross Unrealized Losses	53,444	66,345
Allowance for credit losses	0	0	0
Fair Value	895,775	851,778
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	367,005	399,542
Gross Unrealized Gains	861	4,016
Gross Unrealized Losses	11,794	7,481
Allowance for credit losses	0	7	$ 9
Fair Value	356,072	$ 396,070
Fixed maturities | U.S. Treasury securities and obligations of U.S. government authorities and agencies
Debt Securities, Available-for-sale [Line Items]
Fair Value	1,099,241
Fixed maturities | Obligations of U.S. states and their political subdivisions
Debt Securities, Available-for-sale [Line Items]
Fair Value	541,066
Fixed maturities | Foreign government securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	310,334
Fixed maturities | Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	3,750,663
Fixed maturities | Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	895,775
Fixed maturities | Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	$ 356,072