Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - Fixed maturities - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	$ 8,744,794	$ 10,575,622
Less than Twelve Months, Gross Unrealized Losses	364,781	925,410
Twelve Months or More, Fair Value	12,645,879	5,707,596
Twelve Months or More, Gross Unrealized Losses	2,566,037	1,428,069
Total, Fair Value	21,390,673	16,283,218
Total, Gross Unrealized Losses	2,930,818	2,353,479
U.S. Treasury securities and obligations of U.S. government authorities and agencies
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	97,348	212,991
Less than Twelve Months, Gross Unrealized Losses	3,963	46,928
Twelve Months or More, Fair Value	191,991	62,630
Twelve Months or More, Gross Unrealized Losses	95,548	25,928
Total, Fair Value	289,339	275,621
Total, Gross Unrealized Losses	99,511	72,856
Obligations of U.S. states and their political subdivisions
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	448,520	307,734
Less than Twelve Months, Gross Unrealized Losses	24,252	16,851
Twelve Months or More, Fair Value	212,638	61,915
Twelve Months or More, Gross Unrealized Losses	38,496	14,108
Total, Fair Value	661,158	369,649
Total, Gross Unrealized Losses	62,748	30,959
Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	36,806	139,577
Less than Twelve Months, Gross Unrealized Losses	3,030	19,435
Twelve Months or More, Fair Value	223,995	111,371
Twelve Months or More, Gross Unrealized Losses	72,459	39,205
Total, Fair Value	260,801	250,948
Total, Gross Unrealized Losses	75,489	58,640
U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	3,555,490	3,873,275
Less than Twelve Months, Gross Unrealized Losses	173,174	389,937
Twelve Months or More, Fair Value	4,847,346	1,979,725
Twelve Months or More, Gross Unrealized Losses	1,140,717	602,208
Total, Fair Value	8,402,836	5,853,000
Total, Gross Unrealized Losses	1,313,891	992,145
U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	1,574,199	2,506,932
Less than Twelve Months, Gross Unrealized Losses	66,338	157,853
Twelve Months or More, Fair Value	2,620,568	948,686
Twelve Months or More, Gross Unrealized Losses	361,085	177,352
Total, Fair Value	4,194,767	3,455,618
Total, Gross Unrealized Losses	427,423	335,205
Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	588,547	548,083
Less than Twelve Months, Gross Unrealized Losses	15,738	40,508
Twelve Months or More, Fair Value	951,916	596,437
Twelve Months or More, Gross Unrealized Losses	174,194	122,856
Total, Fair Value	1,540,463	1,144,520
Total, Gross Unrealized Losses	189,932	163,364
Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	1,426,572	1,772,413
Less than Twelve Months, Gross Unrealized Losses	58,934	199,124
Twelve Months or More, Fair Value	2,293,089	1,479,608
Twelve Months or More, Gross Unrealized Losses	563,488	390,029
Total, Fair Value	3,719,661	3,252,021
Total, Gross Unrealized Losses	622,422	589,153
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	580,841	625,710
Less than Twelve Months, Gross Unrealized Losses	7,610	15,146
Twelve Months or More, Fair Value	599,359	289,581
Twelve Months or More, Gross Unrealized Losses	15,592	17,431
Total, Fair Value	1,180,200	915,291
Total, Gross Unrealized Losses	23,202	32,577
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	177,533	459,186
Less than Twelve Months, Gross Unrealized Losses	3,454	30,408
Twelve Months or More, Fair Value	584,984	176,349
Twelve Months or More, Gross Unrealized Losses	90,444	38,693
Total, Fair Value	762,517	635,535
Total, Gross Unrealized Losses	93,898	69,101
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	258,938	129,721
Less than Twelve Months, Gross Unrealized Losses	8,288	9,220
Twelve Months or More, Fair Value	119,993	1,294
Twelve Months or More, Gross Unrealized Losses	14,014	259
Total, Fair Value	378,931	131,015
Total, Gross Unrealized Losses	$ 22,302	$ 9,479