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Investments - Summary of Realized Gain and Loss and Investments in Unrealized Loss Position (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2024
Mar. 31, 2023
Dec. 31, 2023
Investments, Debt and Equity Securities [Abstract]      
Realized gross gains $ 3 $ 869  
Realized gross losses (1,143) $ (1,357)  
Fair Value      
Less than 12 months 1,015,494   $ 550,525
12 months or more 3,034,477   3,200,767
Total 4,049,971   3,751,292
Gross Unrealized Losses      
Less than 12 months (8,394)   (5,759)
12 months or more (346,071)   (330,678)
Total (354,465)   (336,437)
U.S. Treasury securities      
Fair Value      
Less than 12 months 516,852   139,398
12 months or more 318,693   355,921
Total 835,545   495,319
Gross Unrealized Losses      
Less than 12 months (2,708)   (1,075)
12 months or more (18,308)   (18,053)
Total (21,016)   (19,128)
U.S. agency securities      
Fair Value      
Less than 12 months 1,573   5,572
12 months or more 0   1,623
Total 1,573   7,195
Gross Unrealized Losses      
Less than 12 months (1)   (2)
12 months or more 0   (2)
Total (1)   (4)
U.S. agency mortgage-backed securities      
Fair Value      
Less than 12 months 92,502   129,359
12 months or more 683,424   654,018
Total 775,926   783,377
Gross Unrealized Losses      
Less than 12 months (1,363)   (1,616)
12 months or more (113,182)   (100,383)
Total (114,545)   (101,999)
Municipal debt securities      
Fair Value      
Less than 12 months 62,169   59,301
12 months or more 318,055   297,039
Total 380,224   356,340
Gross Unrealized Losses      
Less than 12 months (882)   (987)
12 months or more (44,309)   (43,462)
Total (45,191)   (44,449)
Non-U.S. government securities:      
Fair Value      
Less than 12 months 0   0
12 months or more 66,395   67,447
Total 66,395   67,447
Gross Unrealized Losses      
Less than 12 months 0   0
12 months or more (11,048)   (10,069)
Total (11,048)   (10,069)
Corporate debt securities      
Fair Value      
Less than 12 months 202,873   119,764
12 months or more 888,500   905,606
Total 1,091,373   1,025,370
Gross Unrealized Losses      
Less than 12 months (2,606)   (733)
12 months or more (92,059)   (87,170)
Total (94,665)   (87,903)
Residential and commercial mortgage securities      
Fair Value      
Less than 12 months 20,677   31,936
12 months or more 473,205   459,789
Total 493,882   491,725
Gross Unrealized Losses      
Less than 12 months (323)   (999)
12 months or more (51,978)   (52,510)
Total (52,301)   (53,509)
Asset-backed securities      
Fair Value      
Less than 12 months 118,848   65,195
12 months or more 286,205   459,324
Total 405,053   524,519
Gross Unrealized Losses      
Less than 12 months (511)   (347)
12 months or more (15,187)   (19,029)
Total $ (15,698)   $ (19,376)