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Investments - Summary of Realized Gain and Loss and Investments in Unrealized Loss Position (Details) - USD ($)
 3 Months Ended
Mar. 31, 2023
Mar. 31, 2022
Dec. 31, 2022
Investments, Debt and Equity Securities [Abstract]      
Realized gross gains $ 869,000 $ 12,576,000  
Realized gross losses 1,357,000 13,091,000  
Impairment loss 0 $ 6,837,000  
Fair Value      
Less than 12 months 1,297,817,000   $ 2,474,361,000
12 months or more 2,765,938,000   1,586,877,000
Total 4,063,755,000   4,061,238,000
Gross Unrealized Losses      
Less than 12 months (32,049,000)   (198,613,000)
12 months or more (354,188,000)   (248,891,000)
Total (386,237,000)   (447,504,000)
U.S. Treasury securities      
Fair Value      
Less than 12 months 184,036,000   321,848,000
12 months or more 256,893,000   169,795,000
Total 440,929,000   491,643,000
Gross Unrealized Losses      
Less than 12 months (2,069,000)   (12,381,000)
12 months or more (20,257,000)   (15,695,000)
Total (22,326,000)   (28,076,000)
U.S. agency securities      
Fair Value      
Less than 12 months 11,111,000   7,117,000
12 months or more 0   0
Total 11,111,000   7,117,000
Gross Unrealized Losses      
Less than 12 months (14,000)   (8,000)
12 months or more 0   0
Total (14,000)   (8,000)
U.S. agency mortgage-backed securities      
Fair Value      
Less than 12 months 144,039,000   351,310,000
12 months or more 643,734,000   415,743,000
Total 787,773,000   767,053,000
Gross Unrealized Losses      
Less than 12 months (3,586,000)   (34,193,000)
12 months or more (98,221,000)   (80,997,000)
Total (101,807,000)   (115,190,000)
Municipal debt securities      
Fair Value      
Less than 12 months 150,068,000   335,784,000
12 months or more 244,856,000   64,766,000
Total 394,924,000   400,550,000
Gross Unrealized Losses      
Less than 12 months (2,840,000)   (41,620,000)
12 months or more (44,109,000)   (19,634,000)
Total (46,949,000)   (61,254,000)
Non-U.S. government securities:      
Fair Value      
Less than 12 months 36,274,000   48,071,000
12 months or more 26,744,000   14,328,000
Total 63,018,000   62,399,000
Gross Unrealized Losses      
Less than 12 months (472,000)   (2,914,000)
12 months or more (6,116,000)   (4,338,000)
Total (6,588,000)   (7,252,000)
Corporate debt securities      
Fair Value      
Less than 12 months 467,974,000   811,217,000
12 months or more 774,083,000   421,307,000
Total 1,242,057,000   1,232,524,000
Gross Unrealized Losses      
Less than 12 months (11,248,000)   (69,415,000)
12 months or more (104,839,000)   (63,972,000)
Total (116,087,000)   (133,387,000)
Residential and commercial mortgage securities      
Fair Value      
Less than 12 months 136,012,000   265,934,000
12 months or more 395,814,000   242,366,000
Total 531,826,000   508,300,000
Gross Unrealized Losses      
Less than 12 months (7,065,000)   (22,628,000)
12 months or more (52,736,000)   (43,853,000)
Total (59,801,000)   (66,481,000)
Asset-backed securities      
Fair Value      
Less than 12 months 168,303,000   333,080,000
12 months or more 423,814,000   258,572,000
Total 592,117,000   591,652,000
Gross Unrealized Losses      
Less than 12 months (4,755,000)   (15,454,000)
12 months or more (27,910,000)   (20,402,000)
Total $ (32,665,000)   $ (35,856,000)