Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - USD ($) $ in Thousands	Jun. 30, 2025	Dec. 31, 2024
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	$ 4,758,369	$ 10,716,329
Less than Twelve Months, Gross Unrealized Losses	128,516	267,215
Twelve Months or More, Fair Value	11,184,550	11,256,350
Twelve Months or More, Gross Unrealized Losses	1,582,237	1,893,641
Total, Fair Value	15,942,919	21,972,679
Total, Gross Unrealized Losses	1,710,753	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	287,270	377,531
Less than Twelve Months, Gross Unrealized Losses	12,140	13,829
Twelve Months or More, Fair Value	242,600	238,723
Twelve Months or More, Gross Unrealized Losses	90,735	94,915
Total, Fair Value	529,870	616,254
Total, Gross Unrealized Losses	102,875	108,744
Obligations of U.S. states and their political subdivisions
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	153,557	226,731
Less than Twelve Months, Gross Unrealized Losses	10,293	5,019
Twelve Months or More, Fair Value	236,302	212,060
Twelve Months or More, Gross Unrealized Losses	34,672	25,324
Total, Fair Value	389,859	438,791
Total, Gross Unrealized Losses	44,965	30,343
Foreign government securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	63,036	118,168
Less than Twelve Months, Gross Unrealized Losses	1,107	2,615
Twelve Months or More, Fair Value	170,313	171,166
Twelve Months or More, Gross Unrealized Losses	48,212	49,851
Total, Fair Value	233,349	289,334
Total, Gross Unrealized Losses	49,319	52,466
U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	2,075,023	4,320,552
Less than Twelve Months, Gross Unrealized Losses	72,936	105,145
Twelve Months or More, Fair Value	4,696,782	4,677,336
Twelve Months or More, Gross Unrealized Losses	804,885	852,171
Total, Fair Value	6,771,805	8,997,888
Total, Gross Unrealized Losses	877,821	957,316
U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	1,072,321	1,999,008
Less than Twelve Months, Gross Unrealized Losses	12,883	41,931
Twelve Months or More, Fair Value	2,393,238	2,379,755
Twelve Months or More, Gross Unrealized Losses	191,818	259,489
Total, Fair Value	3,465,559	4,378,763
Total, Gross Unrealized Losses	204,701	301,420
Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	305,143	1,088,644
Less than Twelve Months, Gross Unrealized Losses	6,871	20,465
Twelve Months or More, Fair Value	711,489	716,172
Twelve Months or More, Gross Unrealized Losses	98,027	106,294
Total, Fair Value	1,016,632	1,804,816
Total, Gross Unrealized Losses	104,898	126,759
Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	457,994	1,977,169
Less than Twelve Months, Gross Unrealized Losses	9,918	69,399
Twelve Months or More, Fair Value	2,019,364	2,107,705
Twelve Months or More, Gross Unrealized Losses	258,549	440,330
Total, Fair Value	2,477,358	4,084,874
Total, Gross Unrealized Losses	268,467	509,729
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	281,356	363,744
Less than Twelve Months, Gross Unrealized Losses	1,938	5,510
Twelve Months or More, Fair Value	118,164	140,090
Twelve Months or More, Gross Unrealized Losses	6,839	3,331
Total, Fair Value	399,520	503,834
Total, Gross Unrealized Losses	8,777	8,841
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	34,380	101,821
Less than Twelve Months, Gross Unrealized Losses	105	1,356
Twelve Months or More, Fair Value	474,854	489,490
Twelve Months or More, Gross Unrealized Losses	41,282	52,088
Total, Fair Value	509,234	591,311
Total, Gross Unrealized Losses	41,387	53,444
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	28,289	142,961
Less than Twelve Months, Gross Unrealized Losses	325	1,946
Twelve Months or More, Fair Value	121,444	123,853
Twelve Months or More, Gross Unrealized Losses	7,218	9,848
Total, Fair Value	149,733	266,814
Total, Gross Unrealized Losses	$ 7,543	$ 11,794