Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - USD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	$ 10,716,329	$ 2,492,369
Less than Twelve Months, Gross Unrealized Losses	267,215	39,568
Twelve Months or More Fair Value	11,256,350	13,312,102
Twelve Months or More, Gross Unrealized Losses	1,893,641	1,699,993
Total, Fair Value	21,972,679	15,804,471
Total, Gross Unrealized Losses	2,160,856	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	377,531	98,174
Less than Twelve Months, Gross Unrealized Losses	13,829	945
Twelve Months or More Fair Value	238,723	214,889
Twelve Months or More, Gross Unrealized Losses	94,915	72,563
Total, Fair Value	616,254	313,063
Total, Gross Unrealized Losses	108,744	73,508
Obligations of U.S. states and their political subdivisions
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	226,731	83,729
Less than Twelve Months, Gross Unrealized Losses	5,019	293
Twelve Months or More Fair Value	212,060	218,375
Twelve Months or More, Gross Unrealized Losses	25,324	18,224
Total, Fair Value	438,791	302,104
Total, Gross Unrealized Losses	30,343	18,517
Foreign government securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	118,168	10,226
Less than Twelve Months, Gross Unrealized Losses	2,615	116
Twelve Months or More Fair Value	171,166	233,757
Twelve Months or More, Gross Unrealized Losses	49,851	50,568
Total, Fair Value	289,334	243,983
Total, Gross Unrealized Losses	52,466	50,684
U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	4,320,552	782,904
Less than Twelve Months, Gross Unrealized Losses	105,145	10,009
Twelve Months or More Fair Value	4,677,336	5,201,353
Twelve Months or More, Gross Unrealized Losses	852,171	750,265
Total, Fair Value	8,997,888	5,984,257
Total, Gross Unrealized Losses	957,316	760,274
U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	1,999,008	707,674
Less than Twelve Months, Gross Unrealized Losses	41,931	16,613
Twelve Months or More Fair Value	2,379,755	2,794,697
Twelve Months or More, Gross Unrealized Losses	259,489	238,181
Total, Fair Value	4,378,763	3,502,371
Total, Gross Unrealized Losses	301,420	254,794
Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	1,088,644	92,955
Less than Twelve Months, Gross Unrealized Losses	20,465	1,063
Twelve Months or More Fair Value	716,172	948,963
Twelve Months or More, Gross Unrealized Losses	106,294	114,169
Total, Fair Value	1,804,816	1,041,918
Total, Gross Unrealized Losses	126,759	115,232
Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	1,977,169	429,212
Less than Twelve Months, Gross Unrealized Losses	69,399	8,035
Twelve Months or More Fair Value	2,107,705	2,461,367
Twelve Months or More, Gross Unrealized Losses	440,330	373,180
Total, Fair Value	4,084,874	2,890,579
Total, Gross Unrealized Losses	509,729	381,215
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	363,744	208,970
Less than Twelve Months, Gross Unrealized Losses	5,510	1,761
Twelve Months or More Fair Value	140,090	532,814
Twelve Months or More, Gross Unrealized Losses	3,331	9,750
Total, Fair Value	503,834	741,784
Total, Gross Unrealized Losses	8,841	11,511
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	101,821	42,621
Less than Twelve Months, Gross Unrealized Losses	1,356	298
Twelve Months or More Fair Value	489,490	580,931
Twelve Months or More, Gross Unrealized Losses	52,088	66,047
Total, Fair Value	591,311	623,552
Total, Gross Unrealized Losses	53,444	66,345
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	142,961	35,904
Less than Twelve Months, Gross Unrealized Losses	1,946	435
Twelve Months or More Fair Value	123,853	124,956
Twelve Months or More, Gross Unrealized Losses	9,848	7,046
Total, Fair Value	266,814	160,860
Total, Gross Unrealized Losses	$ 11,794	$ 7,481