Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	$ 2,492,369	$ 10,575,622
Less than Twelve Months, Gross Unrealized Losses	39,568	925,410
Twelve Months or More Fair Value	13,312,102	5,707,596
Twelve Months or More, Gross Unrealized Losses	1,699,993	1,428,069
Total, Fair Value	15,804,471	16,283,218
Total, Gross Unrealized Losses	1,739,561	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	98,174	212,991
Less than Twelve Months, Gross Unrealized Losses	945	46,928
Twelve Months or More Fair Value	214,889	62,630
Twelve Months or More, Gross Unrealized Losses	72,563	25,928
Total, Fair Value	313,063	275,621
Total, Gross Unrealized Losses	73,508	72,856
Obligations of U.S. states and their political subdivisions
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	83,729	307,734
Less than Twelve Months, Gross Unrealized Losses	293	16,851
Twelve Months or More Fair Value	218,375	61,915
Twelve Months or More, Gross Unrealized Losses	18,224	14,108
Total, Fair Value	302,104	369,649
Total, Gross Unrealized Losses	18,517	30,959
Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	10,226	139,577
Less than Twelve Months, Gross Unrealized Losses	116	19,435
Twelve Months or More Fair Value	233,757	111,371
Twelve Months or More, Gross Unrealized Losses	50,568	39,205
Total, Fair Value	243,983	250,948
Total, Gross Unrealized Losses	50,684	58,640
U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	782,904	3,873,275
Less than Twelve Months, Gross Unrealized Losses	10,009	389,937
Twelve Months or More Fair Value	5,201,353	1,979,725
Twelve Months or More, Gross Unrealized Losses	750,265	602,208
Total, Fair Value	5,984,257	5,853,000
Total, Gross Unrealized Losses	760,274	992,145
U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	707,674	2,506,932
Less than Twelve Months, Gross Unrealized Losses	16,613	157,853
Twelve Months or More Fair Value	2,794,697	948,686
Twelve Months or More, Gross Unrealized Losses	238,181	177,352
Total, Fair Value	3,502,371	3,455,618
Total, Gross Unrealized Losses	254,794	335,205
Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	92,955	548,083
Less than Twelve Months, Gross Unrealized Losses	1,063	40,508
Twelve Months or More Fair Value	948,963	596,437
Twelve Months or More, Gross Unrealized Losses	114,169	122,856
Total, Fair Value	1,041,918	1,144,520
Total, Gross Unrealized Losses	115,232	163,364
Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	429,212	1,772,413
Less than Twelve Months, Gross Unrealized Losses	8,035	199,124
Twelve Months or More Fair Value	2,461,367	1,479,608
Twelve Months or More, Gross Unrealized Losses	373,180	390,029
Total, Fair Value	2,890,579	3,252,021
Total, Gross Unrealized Losses	381,215	589,153
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	208,970	625,710
Less than Twelve Months, Gross Unrealized Losses	1,761	15,146
Twelve Months or More Fair Value	532,814	289,581
Twelve Months or More, Gross Unrealized Losses	9,750	17,431
Total, Fair Value	741,784	915,291
Total, Gross Unrealized Losses	11,511	32,577
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	42,621	459,186
Less than Twelve Months, Gross Unrealized Losses	298	30,408
Twelve Months or More Fair Value	580,931	176,349
Twelve Months or More, Gross Unrealized Losses	66,047	38,693
Total, Fair Value	623,552	635,535
Total, Gross Unrealized Losses	66,345	69,101
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	35,904	129,721
Less than Twelve Months, Gross Unrealized Losses	435	9,220
Twelve Months or More Fair Value	124,956	1,294
Twelve Months or More, Gross Unrealized Losses	7,046	259
Total, Fair Value	160,860	131,015
Total, Gross Unrealized Losses	$ 7,481	$ 9,479