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Investments - Schedule of Realized Gain and Loss and Investments in Unrealized Loss Position (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2025
Jun. 30, 2024
Jun. 30, 2025
Jun. 30, 2024
Dec. 31, 2024
Investments, Debt and Equity Securities [Abstract]          
Realized gross gains $ 19 $ 51 $ 19 $ 55  
Realized gross losses (148) (692) (329) (1,836)  
Impairment loss 0 $ (523) 0 $ (523)  
Fair Value          
Less than 12 months 1,354,340   1,354,340   $ 1,954,295
12 months or more 2,301,705   2,301,705   2,567,865
Total 3,656,045   3,656,045   4,522,160
Gross Unrealized Losses          
Less than 12 months (27,896)   (27,896)   (50,014)
12 months or more (242,310)   (242,310)   (308,438)
Total (270,206)   (270,206)   (358,452)
U.S. Treasury securities          
Fair Value          
Less than 12 months 33,700   33,700   141,789
12 months or more 205,869   205,869   279,143
Total 239,569   239,569   420,932
Gross Unrealized Losses          
Less than 12 months (201)   (201)   (1,620)
12 months or more (8,636)   (8,636)   (13,268)
Total (8,837)   (8,837)   (14,888)
U.S. agency mortgage-backed securities          
Fair Value          
Less than 12 months 321,777   321,777   476,695
12 months or more 606,917   606,917   629,036
Total 928,694   928,694   1,105,731
Gross Unrealized Losses          
Less than 12 months (5,139)   (5,139)   (13,028)
12 months or more (90,696)   (90,696)   (110,837)
Total (95,835)   (95,835)   (123,865)
Municipal debt securities          
Fair Value          
Less than 12 months 203,037   203,037   204,810
12 months or more 281,737   281,737   282,312
Total 484,774   484,774   487,122
Gross Unrealized Losses          
Less than 12 months (7,028)   (7,028)   (5,688)
12 months or more (39,965)   (39,965)   (43,422)
Total (46,993)   (46,993)   (49,110)
Non-U.S. government securities          
Fair Value          
Less than 12 months 15,049   15,049   4,146
12 months or more 47,117   47,117   65,652
Total 62,166   62,166   69,798
Gross Unrealized Losses          
Less than 12 months (4,027)   (4,027)   (262)
12 months or more (6,068)   (6,068)   (11,571)
Total (10,095)   (10,095)   (11,833)
Corporate debt securities:          
Fair Value          
Less than 12 months 524,650   524,650   898,449
12 months or more 645,501   645,501   725,148
Total 1,170,151   1,170,151   1,623,597
Gross Unrealized Losses          
Less than 12 months (8,344)   (8,344)   (25,547)
12 months or more (60,958)   (60,958)   (80,118)
Total (69,302)   (69,302)   (105,665)
Residential and commercial mortgage securities          
Fair Value          
Less than 12 months 25,314   25,314   39,931
12 months or more 395,500   395,500   421,891
Total 420,814   420,814   461,822
Gross Unrealized Losses          
Less than 12 months (327)   (327)   (1,168)
12 months or more (30,653)   (30,653)   (40,886)
Total (30,980)   (30,980)   (42,054)
Asset-backed securities          
Fair Value          
Less than 12 months 230,813   230,813   188,475
12 months or more 119,064   119,064   164,683
Total 349,877   349,877   353,158
Gross Unrealized Losses          
Less than 12 months (2,830)   (2,830)   (2,701)
12 months or more (5,334)   (5,334)   (8,336)
Total $ (8,164)   $ (8,164)   $ (11,037)