Investments - Schedule of Realized Gain and Loss and Investments in Unrealized Loss Position (Details) - USD ($) $ in Thousands |
3 Months Ended | 9 Months Ended | |||
|---|---|---|---|---|---|
Sep. 30, 2026 |
Sep. 30, 2025 |
Sep. 30, 2026 |
Sep. 30, 2025 |
Dec. 31, 2025 |
|
| Investments, Debt and Equity Securities [Abstract] | |||||
| Realized gross gains | $ 3 | $ 40 | $ 677 | $ 59 | |
| Realized gross losses | (199) | (425) | (1,131) | (754) | |
| Impairments and changes in expected credit losses | (1,430) | $ (40) | (1,430) | $ (40) | |
| Fair Value | |||||
| Less than 12 months | 3,288,239 | 3,288,239 | $ 458,503 | ||
| 12 months or more | 1,808,436 | 1,808,436 | 2,409,379 | ||
| Total | 5,096,675 | 5,096,675 | 2,867,882 | ||
| Gross Unrealized Losses | |||||
| Less than 12 months | (133,400) | (133,400) | (3,330) | ||
| 12 months or more | (289,447) | (289,447) | (209,963) | ||
| Total | (422,847) | (422,847) | (213,293) | ||
| U.S. Treasury securities | |||||
| Fair Value | |||||
| Less than 12 months | 250,695 | 250,695 | 1,048 | ||
| 12 months or more | 67,149 | 67,149 | 179,121 | ||
| Total | 317,844 | 317,844 | 180,169 | ||
| Gross Unrealized Losses | |||||
| Less than 12 months | (4,748) | (4,748) | (8) | ||
| 12 months or more | (6,636) | (6,636) | (5,968) | ||
| Total | (11,384) | (11,384) | (5,976) | ||
| U.S. agency mortgage-backed securities | |||||
| Fair Value | |||||
| Less than 12 months | 651,356 | 651,356 | 45,415 | ||
| 12 months or more | 469,704 | 469,704 | 638,729 | ||
| Total | 1,121,060 | 1,121,060 | 684,144 | ||
| Gross Unrealized Losses | |||||
| Less than 12 months | (32,901) | (32,901) | (90) | ||
| 12 months or more | (105,211) | (105,211) | (76,660) | ||
| Total | (138,112) | (138,112) | (76,750) | ||
| Municipal debt securities | |||||
| Fair Value | |||||
| Less than 12 months | 290,809 | 290,809 | 74,928 | ||
| 12 months or more | 259,984 | 259,984 | 314,515 | ||
| Total | 550,793 | 550,793 | 389,443 | ||
| Gross Unrealized Losses | |||||
| Less than 12 months | (15,617) | (15,617) | (1,015) | ||
| 12 months or more | (51,446) | (51,446) | (35,210) | ||
| Total | (67,063) | (67,063) | (36,225) | ||
| Non-U.S. government securities | |||||
| Fair Value | |||||
| Less than 12 months | 0 | 0 | 0 | ||
| 12 months or more | 42,169 | 42,169 | 56,024 | ||
| Total | 42,169 | 42,169 | 56,024 | ||
| Gross Unrealized Losses | |||||
| Less than 12 months | 0 | 0 | 0 | ||
| 12 months or more | (11,256) | (11,256) | (8,446) | ||
| Total | (11,256) | (11,256) | (8,446) | ||
| Corporate debt securities: | |||||
| Fair Value | |||||
| Less than 12 months | 1,346,480 | 1,346,480 | 196,571 | ||
| 12 months or more | 553,944 | 553,944 | 706,561 | ||
| Total | 1,900,424 | 1,900,424 | 903,132 | ||
| Gross Unrealized Losses | |||||
| Less than 12 months | (64,970) | (64,970) | (1,106) | ||
| 12 months or more | (78,047) | (78,047) | (53,441) | ||
| Total | (143,017) | (143,017) | (54,547) | ||
| Residential and commercial mortgage securities | |||||
| Fair Value | |||||
| Less than 12 months | 135,269 | 135,269 | 36,552 | ||
| 12 months or more | 302,704 | 302,704 | 364,271 | ||
| Total | 437,973 | 437,973 | 400,823 | ||
| Gross Unrealized Losses | |||||
| Less than 12 months | (5,104) | (5,104) | (240) | ||
| 12 months or more | (24,932) | (24,932) | (23,142) | ||
| Total | (30,036) | (30,036) | (23,382) | ||
| Asset-backed securities | |||||
| Fair Value | |||||
| Less than 12 months | 575,498 | 575,498 | 103,989 | ||
| 12 months or more | 112,782 | 112,782 | 150,158 | ||
| Total | 688,280 | 688,280 | 254,147 | ||
| Gross Unrealized Losses | |||||
| Less than 12 months | (10,052) | (10,052) | (871) | ||
| 12 months or more | (11,919) | (11,919) | (7,096) | ||
| Total | (21,971) | (21,971) | $ (7,967) | ||
| Money market funds | |||||
| Fair Value | |||||
| Less than 12 months | 38,132 | 38,132 | |||
| 12 months or more | 0 | 0 | |||
| Total | 38,132 | 38,132 | |||
| Gross Unrealized Losses | |||||
| Less than 12 months | (8) | (8) | |||
| 12 months or more | 0 | 0 | |||
| Total | $ (8) | $ (8) | |||