Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	$ 5,261,762	$ 2,492,369
Less than Twelve Months, Gross Unrealized Losses	88,071	39,568
Twelve Months or More, Fair Value	13,310,499	13,312,102
Twelve Months or More, Gross Unrealized Losses	1,909,177	1,699,993
Total, Fair Value	18,572,261	15,804,471
Total, Gross Unrealized Losses	1,997,248	1,739,561
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	97,935	98,174
Less than Twelve Months, Gross Unrealized Losses	2,189	945
Twelve Months or More, Fair Value	206,348	214,889
Twelve Months or More, Gross Unrealized Losses	80,956	72,563
Total, Fair Value	304,283	313,063
Total, Gross Unrealized Losses	83,145	73,508
Obligations of U.S. states and their political subdivisions
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	223,854	83,729
Less than Twelve Months, Gross Unrealized Losses	2,020	293
Twelve Months or More, Fair Value	222,390	218,375
Twelve Months or More, Gross Unrealized Losses	21,478	18,224
Total, Fair Value	446,244	302,104
Total, Gross Unrealized Losses	23,498	18,517
Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	25,819	10,226
Less than Twelve Months, Gross Unrealized Losses	805	116
Twelve Months or More, Fair Value	219,833	233,757
Twelve Months or More, Gross Unrealized Losses	56,662	50,568
Total, Fair Value	245,652	243,983
Total, Gross Unrealized Losses	57,467	50,684
U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	1,923,127	782,904
Less than Twelve Months, Gross Unrealized Losses	32,957	10,009
Twelve Months or More, Fair Value	5,440,539	5,201,353
Twelve Months or More, Gross Unrealized Losses	845,953	750,265
Total, Fair Value	7,363,666	5,984,257
Total, Gross Unrealized Losses	878,910	760,274
U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	1,166,202	707,674
Less than Twelve Months, Gross Unrealized Losses	27,298	16,613
Twelve Months or More, Fair Value	2,789,185	2,794,697
Twelve Months or More, Gross Unrealized Losses	266,893	238,181
Total, Fair Value	3,955,387	3,502,371
Total, Gross Unrealized Losses	294,191	254,794
Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	240,006	92,955
Less than Twelve Months, Gross Unrealized Losses	1,856	1,063
Twelve Months or More, Fair Value	949,080	948,963
Twelve Months or More, Gross Unrealized Losses	123,377	114,169
Total, Fair Value	1,189,086	1,041,918
Total, Gross Unrealized Losses	125,233	115,232
Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	948,742	429,212
Less than Twelve Months, Gross Unrealized Losses	17,812	8,035
Twelve Months or More, Fair Value	2,405,290	2,461,367
Twelve Months or More, Gross Unrealized Losses	434,601	373,180
Total, Fair Value	3,354,032	2,890,579
Total, Gross Unrealized Losses	452,413	381,215
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	552,452	208,970
Less than Twelve Months, Gross Unrealized Losses	2,424	1,761
Twelve Months or More, Fair Value	342,617	532,814
Twelve Months or More, Gross Unrealized Losses	6,028	9,750
Total, Fair Value	895,069	741,784
Total, Gross Unrealized Losses	8,452	11,511
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	22,085	42,621
Less than Twelve Months, Gross Unrealized Losses	39	298
Twelve Months or More, Fair Value	585,189	580,931
Twelve Months or More, Gross Unrealized Losses	63,519	66,047
Total, Fair Value	607,274	623,552
Total, Gross Unrealized Losses	63,558	66,345
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	61,540	35,904
Less than Twelve Months, Gross Unrealized Losses	671	435
Twelve Months or More, Fair Value	150,028	124,956
Twelve Months or More, Gross Unrealized Losses	9,710	7,046
Total, Fair Value	211,568	160,860
Total, Gross Unrealized Losses	$ 10,381	$ 7,481