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Investments - Summary of Fixed Maturity Securities Classified as Available-for-Sale (Details) - USD ($) $ in Thousands | Mar. 31, 2025 | Dec. 31, 2024
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost | $ 5,992,370 | $ 5,876,502
Gross unrealized gains | 37,418 | 20,711
Gross unrealized losses | (214,178) | (272,440)
Allowance for credit losses | 273 | 0
Fair value | 5,815,337 | 5,624,773
Total investments |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost | 5,996,062 | 5,879,872
Gross unrealized gains | 37,422 | 20,712
Gross unrealized losses | (214,178) | (272,444)
Allowance for credit losses | 273 |
Fair value | 5,819,033 | 5,628,140
U.S. government, agencies and GSEs |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost | 287,643 | 281,858
Gross unrealized gains | 2,800 | 590
Gross unrealized losses | (1,435) | (5,085)
Allowance for credit losses | 0 |
Fair value | 289,008 | 277,363
State and political subdivisions |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost | 540,495 | 541,715
Gross unrealized gains | 1,571 | 1,388
Gross unrealized losses | (65,925) | (75,627)
Allowance for credit losses | 0 |
Fair value | 476,141 | 467,476
Non-U.S. government |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost | 113,342 | 85,566
Gross unrealized gains | 780 | 94
Gross unrealized losses | (969) | (1,858)
Allowance for credit losses | 0 |
Fair value | 113,153 | 83,802
U.S. corporate |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost | 2,978,401 | 2,943,900
Gross unrealized gains | 20,555 | 11,363
Gross unrealized losses | (98,751) | (129,584)
Allowance for credit losses | 0 |
Fair value | 2,900,205 | 2,825,679
Non-U.S. corporate |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost | 786,865 | 800,141
Gross unrealized gains | 5,589 | 2,533
Gross unrealized losses | (23,228) | (30,050)
Allowance for credit losses | 273 |
Fair value | 768,953 | 772,624
Residential mortgage-backed |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost | 111,660 | 8,394
Gross unrealized gains | 505 | 27
Gross unrealized losses | (482) | (57)
Allowance for credit losses | 0 |
Fair value | 111,683 | 8,364
Other asset-backed |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost | 1,153,367 | 1,214,928
Gross unrealized gains | 5,513 | 4,716
Gross unrealized losses | (23,388) | (30,179)
Allowance for credit losses | 0 |
Fair value | 1,135,492 | 1,189,465
Short-term investments |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost | 3,692 | 3,370
Gross unrealized gains | 4 | 1
Gross unrealized losses | 0 | (4)
Allowance for credit losses | 0 |
Fair value | 3,696 | $ 3,367
Commercial mortgage-backed |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost | 20,597 |
Gross unrealized gains | 105 |
Gross unrealized losses | 0 |
Allowance for credit losses | 0 |
Fair value | $ 20,702 |

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