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Investments - Summary of Realized Gain and Loss and Investments in Unrealized Loss Position (Details) - USD ($)
 3 Months Ended
Mar. 31, 2022
Mar. 31, 2021
Dec. 31, 2021
Investments, Debt and Equity Securities [Abstract]      
Realized gross gains $ 12,576,000 $ 750,000  
Realized gross losses (13,091,000) (109,000)  
Impairment loss (6,837,000) $ 0  
Fair Value      
Less than 12 months 2,919,148,000   $ 2,033,809,000
12 months or more 526,703,000   152,687,000
Total 3,445,851,000   2,186,496,000
Gross Unrealized Losses      
Less than 12 months (135,406,000)   (28,074,000)
12 months or more (48,895,000)   (5,393,000)
Total (184,301,000)   (33,467,000)
U.S. Treasury securities      
Fair Value      
Less than 12 months 303,598,000   207,122,000
12 months or more 67,009,000   28,012,000
Total 370,607,000   235,134,000
Gross Unrealized Losses      
Less than 12 months (10,257,000)   (2,170,000)
12 months or more (3,921,000)   (796,000)
Total (14,178,000)   (2,966,000)
U.S. agency mortgage-backed securities      
Fair Value      
Less than 12 months 614,532,000   582,108,000
12 months or more 191,880,000   26,131,000
Total 806,412,000   608,239,000
Gross Unrealized Losses      
Less than 12 months (31,273,000)   (9,414,000)
12 months or more (17,382,000)   (699,000)
Total (48,655,000)   (10,113,000)
Municipal debt securities      
Fair Value      
Less than 12 months 301,894,000   91,719,000
12 months or more 6,694,000   312,000
Total 308,588,000   92,031,000
Gross Unrealized Losses      
Less than 12 months (21,844,000)   (1,281,000)
12 months or more (646,000)   (6,000)
Total (22,490,000)   (1,287,000)
Non-U.S. government securities:      
Fair Value      
Less than 12 months 16,001,000   22,986,000
12 months or more 14,356,000   0
Total 30,357,000   22,986,000
Gross Unrealized Losses      
Less than 12 months (915,000)   (855,000)
12 months or more (1,850,000)   0
Total (2,765,000)   (855,000)
Corporate debt securities      
Fair Value      
Less than 12 months 833,120,000   522,120,000
12 months or more 125,507,000   46,875,000
Total 958,627,000   568,995,000
Gross Unrealized Losses      
Less than 12 months (42,805,000)   (7,200,000)
12 months or more (15,568,000)   (2,265,000)
Total (58,373,000)   (9,465,000)
Residential and commercial mortgage securities      
Fair Value      
Less than 12 months 356,130,000   268,617,000
12 months or more 91,270,000   38,256,000
Total 447,400,000   306,873,000
Gross Unrealized Losses      
Less than 12 months (18,432,000)   (5,200,000)
12 months or more (8,982,000)   (1,467,000)
Total (27,414,000)   (6,667,000)
Asset-backed securities      
Fair Value      
Less than 12 months 493,873,000   339,137,000
12 months or more 29,987,000   13,101,000
Total 523,860,000   352,238,000
Gross Unrealized Losses      
Less than 12 months (9,880,000)   (1,954,000)
12 months or more (546,000)   (160,000)
Total $ (10,426,000)   $ (2,114,000)