Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	$ 2,518,140	$ 10,716,329
Less than Twelve Months, Gross Unrealized Losses	44,131	267,215
Twelve Months or More, Fair Value	12,015,813	11,256,350
Twelve Months or More, Gross Unrealized Losses	1,443,901	1,893,641
Total, Fair Value	14,533,953	21,972,679
Total, Gross Unrealized Losses	1,488,032	2,160,856
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	290,005	377,531
Less than Twelve Months, Gross Unrealized Losses	8,814	13,829
Twelve Months or More, Fair Value	246,689	238,723
Twelve Months or More, Gross Unrealized Losses	87,033	94,915
Total, Fair Value	536,694	616,254
Total, Gross Unrealized Losses	95,847	108,744
Obligations of U.S. states and their political subdivisions
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	85,588	226,731
Less than Twelve Months, Gross Unrealized Losses	3,913	5,019
Twelve Months or More, Fair Value	234,274	212,060
Twelve Months or More, Gross Unrealized Losses	26,736	25,324
Total, Fair Value	319,862	438,791
Total, Gross Unrealized Losses	30,649	30,343
Foreign government securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	474	118,168
Less than Twelve Months, Gross Unrealized Losses	3	2,615
Twelve Months or More, Fair Value	186,119	171,166
Twelve Months or More, Gross Unrealized Losses	40,135	49,851
Total, Fair Value	186,593	289,334
Total, Gross Unrealized Losses	40,138	52,466
U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	946,829	4,320,552
Less than Twelve Months, Gross Unrealized Losses	18,369	105,145
Twelve Months or More, Fair Value	5,130,020	4,677,336
Twelve Months or More, Gross Unrealized Losses	736,691	852,171
Total, Fair Value	6,076,849	8,997,888
Total, Gross Unrealized Losses	755,060	957,316
U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	624,699	1,999,008
Less than Twelve Months, Gross Unrealized Losses	9,738	41,931
Twelve Months or More, Fair Value	2,636,271	2,379,755
Twelve Months or More, Gross Unrealized Losses	171,110	259,489
Total, Fair Value	3,260,970	4,378,763
Total, Gross Unrealized Losses	180,848	301,420
Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	216,460	1,088,644
Less than Twelve Months, Gross Unrealized Losses	1,052	20,465
Twelve Months or More, Fair Value	782,054	716,172
Twelve Months or More, Gross Unrealized Losses	83,367	106,294
Total, Fair Value	998,514	1,804,816
Total, Gross Unrealized Losses	84,419	126,759
Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	144,817	1,977,169
Less than Twelve Months, Gross Unrealized Losses	1,978	69,399
Twelve Months or More, Fair Value	2,159,311	2,107,705
Twelve Months or More, Gross Unrealized Losses	255,167	440,330
Total, Fair Value	2,304,128	4,084,874
Total, Gross Unrealized Losses	257,145	509,729
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	180,417	363,744
Less than Twelve Months, Gross Unrealized Losses	261	5,510
Twelve Months or More, Fair Value	95,008	140,090
Twelve Months or More, Gross Unrealized Losses	3,774	3,331
Total, Fair Value	275,425	503,834
Total, Gross Unrealized Losses	4,035	8,841
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	28,819	101,821
Less than Twelve Months, Gross Unrealized Losses	3	1,356
Twelve Months or More, Fair Value	410,363	489,490
Twelve Months or More, Gross Unrealized Losses	34,339	52,088
Total, Fair Value	439,182	591,311
Total, Gross Unrealized Losses	34,342	53,444
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	32	142,961
Less than Twelve Months, Gross Unrealized Losses	0	1,946
Twelve Months or More, Fair Value	135,704	123,853
Twelve Months or More, Gross Unrealized Losses	5,549	9,848
Total, Fair Value	135,736	266,814
Total, Gross Unrealized Losses	$ 5,549	$ 11,794