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Investments - Summary of Realized Gain and Loss and Investments in Unrealized Loss Position (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2022
Sep. 30, 2021
Sep. 30, 2022
Sep. 30, 2021
Dec. 31, 2021
Investments, Debt and Equity Securities [Abstract]          
Realized gross gains $ 1,650 $ 654 $ 14,397 $ 1,422  
Realized gross losses (1,370) (433) (14,634) (813)  
Impairment loss (105) $ 0 (7,411) $ 0  
Fair Value          
Less than 12 months 3,473,897   3,473,897   $ 2,033,809
12 months or more 838,629   838,629   152,687
Total 4,312,526   4,312,526   2,186,496
Gross Unrealized Losses          
Less than 12 months (352,622)   (352,622)   (28,074)
12 months or more (138,758)   (138,758)   (5,393)
Total (491,380)   (491,380)   (33,467)
U.S. Treasury securities          
Fair Value          
Less than 12 months 361,655   361,655   207,122
12 months or more 161,968   161,968   28,012
Total 523,623   523,623   235,134
Gross Unrealized Losses          
Less than 12 months (15,490)   (15,490)   (2,170)
12 months or more (15,342)   (15,342)   (796)
Total (30,832)   (30,832)   (2,966)
U.S. agency mortgage-backed securities          
Fair Value          
Less than 12 months 508,415   508,415   582,108
12 months or more 243,821   243,821   26,131
Total 752,236   752,236   608,239
Gross Unrealized Losses          
Less than 12 months (74,350)   (74,350)   (9,414)
12 months or more (52,442)   (52,442)   (699)
Total (126,792)   (126,792)   (10,113)
Municipal debt securities          
Fair Value          
Less than 12 months 527,796   527,796   91,719
12 months or more 14,588   14,588   312
Total 542,384   542,384   92,031
Gross Unrealized Losses          
Less than 12 months (64,097)   (64,097)   (1,281)
12 months or more (3,986)   (3,986)   (6)
Total (68,083)   (68,083)   (1,287)
Non-U.S. government securities:          
Fair Value          
Less than 12 months 51,784   51,784   22,986
12 months or more 9,050   9,050   0
Total 60,834   60,834   22,986
Gross Unrealized Losses          
Less than 12 months (5,922)   (5,922)   (855)
12 months or more (3,370)   (3,370)   0
Total (9,292)   (9,292)   (855)
Corporate debt securities          
Fair Value          
Less than 12 months 1,166,786   1,166,786   522,120
12 months or more 143,745   143,745   46,875
Total 1,310,531   1,310,531   568,995
Gross Unrealized Losses          
Less than 12 months (127,688)   (127,688)   (7,200)
12 months or more (28,200)   (28,200)   (2,265)
Total (155,888)   (155,888)   (9,465)
Residential and commercial mortgage securities          
Fair Value          
Less than 12 months 376,356   376,356   268,617
12 months or more 143,118   143,118   38,256
Total 519,474   519,474   306,873
Gross Unrealized Losses          
Less than 12 months (38,046)   (38,046)   (5,200)
12 months or more (25,240)   (25,240)   (1,467)
Total (63,286)   (63,286)   (6,667)
Asset-backed securities          
Fair Value          
Less than 12 months 481,105   481,105   339,137
12 months or more 122,339   122,339   13,101
Total 603,444   603,444   352,238
Gross Unrealized Losses          
Less than 12 months (27,029)   (27,029)   (1,954)
12 months or more (10,178)   (10,178)   (160)
Total $ (37,207)   $ (37,207)   $ (2,114)