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Investments - Schedule of Realized Gain and Loss and Investments in Unrealized Loss Position (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Dec. 31, 2024
Investments, Debt and Equity Securities [Abstract]          
Realized gross gains $ 40 $ 360 $ 59 $ 415  
Realized gross losses (425) (292) (754) (2,128)  
Impairment loss (40) $ 0 (40) $ (523)  
Fair Value          
Less than 12 months 546,646   546,646   $ 1,954,295
12 months or more 2,458,107   2,458,107   2,567,865
Total 3,004,753   3,004,753   4,522,160
Gross Unrealized Losses          
Less than 12 months (8,341)   (8,341)   (50,014)
12 months or more (218,879)   (218,879)   (308,438)
Total (227,220)   (227,220)   (358,452)
U.S. Treasury securities          
Fair Value          
Less than 12 months 15,373   15,373   141,789
12 months or more 206,912   206,912   279,143
Total 222,285   222,285   420,932
Gross Unrealized Losses          
Less than 12 months (97)   (97)   (1,620)
12 months or more (7,031)   (7,031)   (13,268)
Total (7,128)   (7,128)   (14,888)
U.S. agency mortgage-backed securities          
Fair Value          
Less than 12 months 122,399   122,399   476,695
12 months or more 654,260   654,260   629,036
Total 776,659   776,659   1,105,731
Gross Unrealized Losses          
Less than 12 months (1,534)   (1,534)   (13,028)
12 months or more (81,380)   (81,380)   (110,837)
Total (82,914)   (82,914)   (123,865)
Municipal debt securities          
Fair Value          
Less than 12 months 111,110   111,110   204,810
12 months or more 286,020   286,020   282,312
Total 397,130   397,130   487,122
Gross Unrealized Losses          
Less than 12 months (2,420)   (2,420)   (5,688)
12 months or more (35,566)   (35,566)   (43,422)
Total (37,986)   (37,986)   (49,110)
Non-U.S. government securities          
Fair Value          
Less than 12 months 0   0   4,146
12 months or more 57,173   57,173   65,652
Total 57,173   57,173   69,798
Gross Unrealized Losses          
Less than 12 months 0   0   (262)
12 months or more (8,603)   (8,603)   (11,571)
Total (8,603)   (8,603)   (11,833)
Corporate debt securities:          
Fair Value          
Less than 12 months 152,570   152,570   898,449
12 months or more 746,781   746,781   725,148
Total 899,351   899,351   1,623,597
Gross Unrealized Losses          
Less than 12 months (1,704)   (1,704)   (25,547)
12 months or more (54,743)   (54,743)   (80,118)
Total (56,447)   (56,447)   (105,665)
Residential and commercial mortgage securities          
Fair Value          
Less than 12 months 42,064   42,064   39,931
12 months or more 365,795   365,795   421,891
Total 407,859   407,859   461,822
Gross Unrealized Losses          
Less than 12 months (950)   (950)   (1,168)
12 months or more (25,254)   (25,254)   (40,886)
Total (26,204)   (26,204)   (42,054)
Asset-backed securities          
Fair Value          
Less than 12 months 103,130   103,130   188,475
12 months or more 141,166   141,166   164,683
Total 244,296   244,296   353,158
Gross Unrealized Losses          
Less than 12 months (1,636)   (1,636)   (2,701)
12 months or more (6,302)   (6,302)   (8,336)
Total $ (7,938)   $ (7,938)   $ (11,037)