Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - USD ($) $ in Thousands	Mar. 31, 2025	Dec. 31, 2024
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	$ 9,008,882	$ 10,716,329
Less than Twelve Months, Gross Unrealized Losses	175,573	267,215
Twelve Months or More, Fair Value	11,462,335	11,256,350
Twelve Months or More, Gross Unrealized Losses	1,754,173	1,893,641
Total, Fair Value	20,471,217	21,972,679
Total, Gross Unrealized Losses	1,929,746	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	261,400	377,531
Less than Twelve Months, Gross Unrealized Losses	3,536	13,829
Twelve Months or More, Fair Value	247,293	238,723
Twelve Months or More, Gross Unrealized Losses	86,164	94,915
Total, Fair Value	508,693	616,254
Total, Gross Unrealized Losses	89,700	108,744
Obligations of U.S. states and their political subdivisions
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	179,660	226,731
Less than Twelve Months, Gross Unrealized Losses	7,026	5,019
Twelve Months or More, Fair Value	244,108	212,060
Twelve Months or More, Gross Unrealized Losses	28,419	25,324
Total, Fair Value	423,768	438,791
Total, Gross Unrealized Losses	35,445	30,343
Foreign government securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	54,388	118,168
Less than Twelve Months, Gross Unrealized Losses	1,173	2,615
Twelve Months or More, Fair Value	173,897	171,166
Twelve Months or More, Gross Unrealized Losses	47,833	49,851
Total, Fair Value	228,285	289,334
Total, Gross Unrealized Losses	49,006	52,466
U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	2,937,255	4,320,552
Less than Twelve Months, Gross Unrealized Losses	74,371	105,145
Twelve Months or More, Fair Value	4,807,764	4,677,336
Twelve Months or More, Gross Unrealized Losses	818,088	852,171
Total, Fair Value	7,745,019	8,997,888
Total, Gross Unrealized Losses	892,459	957,316
U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	2,004,535	1,999,008
Less than Twelve Months, Gross Unrealized Losses	27,408	41,931
Twelve Months or More, Fair Value	2,416,353	2,379,755
Twelve Months or More, Gross Unrealized Losses	223,102	259,489
Total, Fair Value	4,420,888	4,378,763
Total, Gross Unrealized Losses	250,510	301,420
Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	627,642	1,088,644
Less than Twelve Months, Gross Unrealized Losses	9,733	20,465
Twelve Months or More, Fair Value	740,032	716,172
Twelve Months or More, Gross Unrealized Losses	103,289	106,294
Total, Fair Value	1,367,674	1,804,816
Total, Gross Unrealized Losses	113,022	126,759
Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	1,765,712	1,977,169
Less than Twelve Months, Gross Unrealized Losses	47,356	69,399
Twelve Months or More, Fair Value	2,109,881	2,107,705
Twelve Months or More, Gross Unrealized Losses	388,941	440,330
Total, Fair Value	3,875,593	4,084,874
Total, Gross Unrealized Losses	436,297	509,729
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	1,116,137	363,744
Less than Twelve Months, Gross Unrealized Losses	4,194	5,510
Twelve Months or More, Fair Value	119,537	140,090
Twelve Months or More, Gross Unrealized Losses	6,588	3,331
Total, Fair Value	1,235,674	503,834
Total, Gross Unrealized Losses	10,782	8,841
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	38,559	101,821
Less than Twelve Months, Gross Unrealized Losses	429	1,356
Twelve Months or More, Fair Value	479,676	489,490
Twelve Months or More, Gross Unrealized Losses	44,136	52,088
Total, Fair Value	518,235	591,311
Total, Gross Unrealized Losses	44,565	53,444
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	23,594	142,961
Less than Twelve Months, Gross Unrealized Losses	347	1,946
Twelve Months or More, Fair Value	123,794	123,853
Twelve Months or More, Gross Unrealized Losses	7,613	9,848
Total, Fair Value	147,388	266,814
Total, Gross Unrealized Losses	$ 7,960	$ 11,794