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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, 2024
Jun. 30, 2023
Jun. 30, 2024
Jun. 30, 2023
Dec. 31, 2023
Investments, Debt and Equity Securities [Abstract]          
Realized gross gains $ 51 $ 23 $ 55 $ 892  
Realized gross losses (692) (1,436) (1,836) (2,793)  
Impairment loss (523) $ (176) (523) $ (176)  
Fair Value          
Less than 12 months 1,184,590   1,184,590   $ 550,525
12 months or more 2,697,361   2,697,361   3,200,767
Total 3,881,951   3,881,951   3,751,292
Gross Unrealized Losses          
Less than 12 months (26,402)   (26,402)   (5,759)
12 months or more (333,218)   (333,218)   (330,678)
Total (359,620)   (359,620)   (336,437)
U.S. Treasury securities          
Fair Value          
Less than 12 months 676,520   676,520   139,398
12 months or more 156,302   156,302   355,921
Total 832,822   832,822   495,319
Gross Unrealized Losses          
Less than 12 months (10,015)   (10,015)   (1,075)
12 months or more (10,358)   (10,358)   (18,053)
Total (20,373)   (20,373)   (19,128)
U.S. agency securities          
Fair Value          
Less than 12 months         5,572
12 months or more         1,623
Total         7,195
Gross Unrealized Losses          
Less than 12 months         (2)
12 months or more         (2)
Total         (4)
U.S. agency mortgage-backed securities          
Fair Value          
Less than 12 months 79,991   79,991   129,359
12 months or more 670,718   670,718   654,018
Total 750,709   750,709   783,377
Gross Unrealized Losses          
Less than 12 months (2,181)   (2,181)   (1,616)
12 months or more (116,303)   (116,303)   (100,383)
Total (118,484)   (118,484)   (101,999)
Municipal debt securities          
Fair Value          
Less than 12 months 91,994   91,994   59,301
12 months or more 309,410   309,410   297,039
Total 401,404   401,404   356,340
Gross Unrealized Losses          
Less than 12 months (2,191)   (2,191)   (987)
12 months or more (44,733)   (44,733)   (43,462)
Total (46,924)   (46,924)   (44,449)
Non-U.S. government securities:          
Fair Value          
Less than 12 months 2,999   2,999   0
12 months or more 63,032   63,032   67,447
Total 66,031   66,031   67,447
Gross Unrealized Losses          
Less than 12 months (1,406)   (1,406)   0
12 months or more (9,933)   (9,933)   (10,069)
Total (11,339)   (11,339)   (10,069)
Corporate debt securities:          
Fair Value          
Less than 12 months 256,400   256,400   119,764
12 months or more 812,456   812,456   905,606
Total 1,068,856   1,068,856   1,025,370
Gross Unrealized Losses          
Less than 12 months (7,793)   (7,793)   (733)
12 months or more (89,553)   (89,553)   (87,170)
Total (97,346)   (97,346)   (87,903)
Residential and commercial mortgage securities          
Fair Value          
Less than 12 months 27,485   27,485   31,936
12 months or more 457,287   457,287   459,789
Total 484,772   484,772   491,725
Gross Unrealized Losses          
Less than 12 months (1,716)   (1,716)   (999)
12 months or more (50,253)   (50,253)   (52,510)
Total (51,969)   (51,969)   (53,509)
Asset-backed securities          
Fair Value          
Less than 12 months 49,201   49,201   65,195
12 months or more 228,156   228,156   459,324
Total 277,357   277,357   524,519
Gross Unrealized Losses          
Less than 12 months (1,100)   (1,100)   (347)
12 months or more (12,085)   (12,085)   (19,029)
Total $ (13,185)   $ (13,185)   $ (19,376)