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Investments - Summary of Realized Gain and Loss and Investments in Unrealized Loss Position (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2023
Jun. 30, 2022
Jun. 30, 2023
Jun. 30, 2022
Dec. 31, 2022
Investments, Debt and Equity Securities [Abstract]          
Realized gross gains $ 23 $ 171 $ 892 $ 12,747  
Realized gross losses (1,436) (173) (2,793) (13,264)  
Impairment loss (176) $ (469) (176) $ (7,306)  
Fair Value          
Less than 12 months 1,249,263   1,249,263   $ 2,474,361
12 months or more 3,000,615   3,000,615   1,586,877
Total 4,249,878   4,249,878   4,061,238
Gross Unrealized Losses          
Less than 12 months (29,430)   (29,430)   (198,613)
12 months or more (393,323)   (393,323)   (248,891)
Total (422,753)   (422,753)   (447,504)
U.S. Treasury securities          
Fair Value          
Less than 12 months 274,448   274,448   321,848
12 months or more 293,776   293,776   169,795
Total 568,224   568,224   491,643
Gross Unrealized Losses          
Less than 12 months (4,066)   (4,066)   (12,381)
12 months or more (23,388)   (23,388)   (15,695)
Total (27,454)   (27,454)   (28,076)
U.S. agency securities          
Fair Value          
Less than 12 months 9,671   9,671   7,117
12 months or more 0   0   0
Total 9,671   9,671   7,117
Gross Unrealized Losses          
Less than 12 months (27)   (27)   (8)
12 months or more 0   0   0
Total (27)   (27)   (8)
U.S. agency mortgage-backed securities          
Fair Value          
Less than 12 months 169,514   169,514   351,310
12 months or more 623,847   623,847   415,743
Total 793,361   793,361   767,053
Gross Unrealized Losses          
Less than 12 months (5,925)   (5,925)   (34,193)
12 months or more (108,241)   (108,241)   (80,997)
Total (114,166)   (114,166)   (115,190)
Municipal debt securities          
Fair Value          
Less than 12 months 196,380   196,380   335,784
12 months or more 253,299   253,299   64,766
Total 449,679   449,679   400,550
Gross Unrealized Losses          
Less than 12 months (4,163)   (4,163)   (41,620)
12 months or more (45,758)   (45,758)   (19,634)
Total (49,921)   (49,921)   (61,254)
Non-U.S. government securities:          
Fair Value          
Less than 12 months 15,664   15,664   48,071
12 months or more 34,889   34,889   14,328
Total 50,553   50,553   62,399
Gross Unrealized Losses          
Less than 12 months (411)   (411)   (2,914)
12 months or more (6,481)   (6,481)   (4,338)
Total (6,892)   (6,892)   (7,252)
Corporate debt securities          
Fair Value          
Less than 12 months 399,331   399,331   811,217
12 months or more 856,101   856,101   421,307
Total 1,255,432   1,255,432   1,232,524
Gross Unrealized Losses          
Less than 12 months (10,073)   (10,073)   (69,415)
12 months or more (115,129)   (115,129)   (63,972)
Total (125,202)   (125,202)   (133,387)
Residential and commercial mortgage securities          
Fair Value          
Less than 12 months 60,682   60,682   265,934
12 months or more 443,965   443,965   242,366
Total 504,647   504,647   508,300
Gross Unrealized Losses          
Less than 12 months (3,586)   (3,586)   (22,628)
12 months or more (61,835)   (61,835)   (43,853)
Total (65,421)   (65,421)   (66,481)
Asset-backed securities          
Fair Value          
Less than 12 months 123,573   123,573   333,080
12 months or more 494,738   494,738   258,572
Total 618,311   618,311   591,652
Gross Unrealized Losses          
Less than 12 months (1,179)   (1,179)   (15,454)
12 months or more (32,491)   (32,491)   (20,402)
Total $ (33,670)   $ (33,670)   $ (35,856)