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Investments - Assets Measured at Fair Value (Details) - USD ($) $ in Thousands | Sep. 30, 2025 | Dec. 31, 2024
--- | --- | ---
Assets measured at Fair Value |  |
Available-for-sale fixed income | $ 3,536,484 | $ 3,175,796
Equity securities | 878,872 | 736,191
U.S. government |  |
Assets measured at Fair Value |  |
Available-for-sale fixed income | 399,593 | 515,635
U.S. Agency |  |
Assets measured at Fair Value |  |
Available-for-sale fixed income | 55,483 | 54,338
Non-U.S. government & agency |  |
Assets measured at Fair Value |  |
Available-for-sale fixed income | 12,377 | 7,871
Agency MBS |  |
Assets measured at Fair Value |  |
Available-for-sale fixed income | 604,351 | 396,223
ABS/CMBS/MBS |  |
Assets measured at Fair Value |  |
Available-for-sale fixed income | 563,754 | 410,248
Corporate |  |
Assets measured at Fair Value |  |
Available-for-sale fixed income | 1,490,035 | 1,346,521
Municipal |  |
Assets measured at Fair Value |  |
Available-for-sale fixed income | 410,891 | 444,960
Fair Value Measured on Recurring Basis |  |
Assets measured at Fair Value |  |
Available-for-sale fixed income | 3,536,484 | 3,175,796
Equity securities | 878,872 | 736,191
Total assets at fair value | 4,415,356 | 3,911,987
Fair Value Measured on Recurring Basis | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |
Assets measured at Fair Value |  |
Equity securities | 874,621 | 731,569
Total assets at fair value | 874,621 | 731,569
Fair Value Measured on Recurring Basis | Significant Other Observable Inputs (Level 2) |  |
Assets measured at Fair Value |  |
Available-for-sale fixed income | 3,439,225 | 3,085,293
Total assets at fair value | 3,439,225 | 3,085,293
Fair Value Measured on Recurring Basis | Significant Unobservable Inputs (Level 3) |  |
Assets measured at Fair Value |  |
Available-for-sale fixed income | 97,259 | 90,503
Equity securities | 4,251 | 4,622
Total assets at fair value | 101,510 | 95,125
Fair Value Measured on Recurring Basis | U.S. government |  |
Assets measured at Fair Value |  |
Available-for-sale fixed income | 399,593 | 515,635
Fair Value Measured on Recurring Basis | U.S. government | Significant Other Observable Inputs (Level 2) |  |
Assets measured at Fair Value |  |
Available-for-sale fixed income | 399,593 | 515,635
Fair Value Measured on Recurring Basis | U.S. Agency |  |
Assets measured at Fair Value |  |
Available-for-sale fixed income | 55,483 | 54,338
Fair Value Measured on Recurring Basis | U.S. Agency | Significant Other Observable Inputs (Level 2) |  |
Assets measured at Fair Value |  |
Available-for-sale fixed income | 55,483 | 54,338
Fair Value Measured on Recurring Basis | Non-U.S. government & agency |  |
Assets measured at Fair Value |  |
Available-for-sale fixed income | 12,377 | 7,871
Fair Value Measured on Recurring Basis | Non-U.S. government & agency | Significant Other Observable Inputs (Level 2) |  |
Assets measured at Fair Value |  |
Available-for-sale fixed income | 11,376 | 6,898
Fair Value Measured on Recurring Basis | Non-U.S. government & agency | Significant Unobservable Inputs (Level 3) |  |
Assets measured at Fair Value |  |
Available-for-sale fixed income | 1,001 | 973
Fair Value Measured on Recurring Basis | Agency MBS |  |
Assets measured at Fair Value |  |
Available-for-sale fixed income | 604,351 | 396,223
Fair Value Measured on Recurring Basis | Agency MBS | Significant Other Observable Inputs (Level 2) |  |
Assets measured at Fair Value |  |
Available-for-sale fixed income | 604,351 | 396,223
Fair Value Measured on Recurring Basis | ABS/CMBS/MBS |  |
Assets measured at Fair Value |  |
Available-for-sale fixed income | 563,754 | 410,248
Fair Value Measured on Recurring Basis | ABS/CMBS/MBS | Significant Other Observable Inputs (Level 2) |  |
Assets measured at Fair Value |  |
Available-for-sale fixed income | 563,754 | 410,248
Fair Value Measured on Recurring Basis | Corporate |  |
Assets measured at Fair Value |  |
Available-for-sale fixed income | 1,490,035 | 1,346,521
Fair Value Measured on Recurring Basis | Corporate | Significant Other Observable Inputs (Level 2) |  |
Assets measured at Fair Value |  |
Available-for-sale fixed income | 1,393,777 | 1,256,991
Fair Value Measured on Recurring Basis | Corporate | Significant Unobservable Inputs (Level 3) |  |
Assets measured at Fair Value |  |
Available-for-sale fixed income | 96,258 | 89,530
Fair Value Measured on Recurring Basis | Municipal |  |
Assets measured at Fair Value |  |
Available-for-sale fixed income | 410,891 | 444,960
Fair Value Measured on Recurring Basis | Municipal | Significant Other Observable Inputs (Level 2) |  |
Assets measured at Fair Value |  |
Available-for-sale fixed income | $ 410,891 | $ 444,960

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