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Investments - Schedule of Realized Gain and Loss and Investments in Unrealized Loss Position (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]      
Realized gross gains $ 14,420 $ 4,044 $ 5,608
Realized gross losses 14,864 3,626 2,482
Impairment loss 12,728 0 $ 429
Fair Value      
Less than 12 months 2,474,361 2,033,809  
12 months or more 1,586,877 152,687  
Total 4,061,238 2,186,496  
Gross Unrealized Losses      
Less than 12 months (198,613) (28,074)  
12 months or more (248,891) (5,393)  
Total (447,504) (33,467)  
U.S. Treasury securities      
Fair Value      
Less than 12 months 321,848 207,122  
12 months or more 169,795 28,012  
Total 491,643 235,134  
Gross Unrealized Losses      
Less than 12 months (12,381) (2,170)  
12 months or more (15,695) (796)  
Total (28,076) (2,966)  
U.S. agency securities      
Fair Value      
Less than 12 months 7,117    
12 months or more 0    
Total 7,117    
Gross Unrealized Losses      
Less than 12 months (8)    
12 months or more 0    
Total (8)    
U.S. agency mortgage-backed securities      
Fair Value      
Less than 12 months 351,310 582,108  
12 months or more 415,743 26,131  
Total 767,053 608,239  
Gross Unrealized Losses      
Less than 12 months (34,193) (9,414)  
12 months or more (80,997) (699)  
Total (115,190) (10,113)  
Municipal debt securities      
Fair Value      
Less than 12 months 335,784 91,719  
12 months or more 64,766 312  
Total 400,550 92,031  
Gross Unrealized Losses      
Less than 12 months (41,620) (1,281)  
12 months or more (19,634) (6)  
Total (61,254) (1,287)  
Non-U.S. government securities      
Fair Value      
Less than 12 months 48,071 22,986  
12 months or more 14,328 0  
Total 62,399 22,986  
Gross Unrealized Losses      
Less than 12 months (2,914) (855)  
12 months or more (4,338) 0  
Total (7,252) (855)  
Corporate debt securities      
Fair Value      
Less than 12 months 811,217 522,120  
12 months or more 421,307 46,875  
Total 1,232,524 568,995  
Gross Unrealized Losses      
Less than 12 months (69,415) (7,200)  
12 months or more (63,972) (2,265)  
Total (133,387) (9,465)  
Residential and commercial mortgage securities      
Fair Value      
Less than 12 months 265,934 268,617  
12 months or more 242,366 38,256  
Total 508,300 306,873  
Gross Unrealized Losses      
Less than 12 months (22,628) (5,200)  
12 months or more (43,853) (1,467)  
Total (66,481) (6,667)  
Asset-backed securities      
Fair Value      
Less than 12 months 333,080 339,137  
12 months or more 258,572 13,101  
Total 591,652 352,238  
Gross Unrealized Losses      
Less than 12 months (15,454) (1,954)  
12 months or more (20,402) (160)  
Total $ (35,856) $ (2,114)