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Investments - Schedule of Realized Gain and Loss and Investments in Unrealized Loss Position (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Dec. 31, 2024
Investments, Debt and Equity Securities [Abstract]      
Realized gross gains $ 0 $ 3  
Realized gross losses (181) $ (1,143)  
Fair Value      
Less than 12 months 1,579,473   $ 1,954,295
12 months or more 2,454,644   2,567,865
Total 4,034,117   4,522,160
Gross Unrealized Losses      
Less than 12 months (25,377)   (50,014)
12 months or more (257,009)   (308,438)
Total (282,386)   (358,452)
U.S. Treasury securities      
Fair Value      
Less than 12 months 63,222   141,789
12 months or more 278,244   279,143
Total 341,466   420,932
Gross Unrealized Losses      
Less than 12 months (196)   (1,620)
12 months or more (10,207)   (13,268)
Total (10,403)   (14,888)
U.S. agency mortgage-backed securities      
Fair Value      
Less than 12 months 306,077   476,695
12 months or more 604,998   629,036
Total 911,075   1,105,731
Gross Unrealized Losses      
Less than 12 months (4,678)   (13,028)
12 months or more (86,735)   (110,837)
Total (91,413)   (123,865)
Municipal debt securities      
Fair Value      
Less than 12 months 185,865   204,810
12 months or more 287,275   282,312
Total 473,140   487,122
Gross Unrealized Losses      
Less than 12 months (4,408)   (5,688)
12 months or more (38,798)   (43,422)
Total (43,206)   (49,110)
Non-U.S. government securities:      
Fair Value      
Less than 12 months 0   4,146
12 months or more 61,870   65,652
Total 61,870   69,798
Gross Unrealized Losses      
Less than 12 months 0   (262)
12 months or more (10,468)   (11,571)
Total (10,468)   (11,833)
Corporate debt securities:      
Fair Value      
Less than 12 months 762,984   898,449
12 months or more 665,362   725,148
Total 1,428,346   1,623,597
Gross Unrealized Losses      
Less than 12 months (13,455)   (25,547)
12 months or more (68,024)   (80,118)
Total (81,479)   (105,665)
Residential and commercial mortgage securities      
Fair Value      
Less than 12 months 29,793   39,931
12 months or more 410,962   421,891
Total 440,755   461,822
Gross Unrealized Losses      
Less than 12 months (569)   (1,168)
12 months or more (35,379)   (40,886)
Total (35,948)   (42,054)
Asset-backed securities      
Fair Value      
Less than 12 months 231,532   188,475
12 months or more 145,933   164,683
Total 377,465   353,158
Gross Unrealized Losses      
Less than 12 months (2,071)   (2,701)
12 months or more (7,398)   (8,336)
Total $ (9,469)   $ (11,037)