Investments (Fixed Maturities Securities Excluding Investments Classified as Trading) (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 29,821,440	$ 27,538,066
Gross Unrealized Gains	209,160	335,759
Gross Unrealized Losses	1,997,466	1,740,037
Allowance for Credit Losses	1,650	2,008
Fair Value	28,031,484	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	1,010,188	1,009,937
Gross Unrealized Gains	21,942	38,858
Gross Unrealized Losses	83,145	73,508
Allowance for Credit Losses	0	0
Fair Value	948,985	975,287
Obligations of U.S. states and their political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	752,425	789,856
Gross Unrealized Gains	2,787	5,288
Gross Unrealized Losses	23,498	18,517
Allowance for Credit Losses	0	0
Fair Value	731,714	776,627
Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	323,945	330,830
Gross Unrealized Gains	1,146	1,840
Gross Unrealized Losses	57,467	50,684
Allowance for Credit Losses	0	0
Fair Value	267,624	281,986
U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	10,833,337	10,159,089
Gross Unrealized Gains	51,557	98,047
Gross Unrealized Losses	878,936	760,274
Allowance for Credit Losses	1,232	950
Fair Value	10,004,726	9,495,912
U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,256,562	5,207,699
Gross Unrealized Gains	20,051	37,435
Gross Unrealized Losses	294,191	254,828
Allowance for Credit Losses	185	812
Fair Value	4,982,237	4,989,494
Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,178,852	1,809,347
Gross Unrealized Gains	9,138	12,658
Gross Unrealized Losses	125,425	115,673
Allowance for Credit Losses	233	238
Fair Value	2,062,332	1,706,094
Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,154,303	4,902,391
Gross Unrealized Gains	69,870	109,806
Gross Unrealized Losses	452,413	381,215
Allowance for Credit Losses	0	0
Fair Value	4,771,760	4,630,982
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,024,574	2,016,028
Gross Unrealized Gains	26,682	23,035
Gross Unrealized Losses	8,452	11,512
Allowance for Credit Losses	0	1
Fair Value	3,042,804	2,027,550
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	893,912	913,347
Gross Unrealized Gains	4,163	4,776
Gross Unrealized Losses	63,558	66,345
Allowance for Credit Losses	0	0
Fair Value	834,517	851,778
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	393,342	399,542
Gross Unrealized Gains	1,824	4,016
Gross Unrealized Losses	10,381	7,481
Allowance for Credit Losses	0	7
Fair Value	$ 384,785	$ 396,070