Investments - Assets Measured at Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2026	Dec. 31, 2025
Assets measured at Fair Value
Available-for-sale fixed income	$ 3,617,942	$ 3,533,336
Equity securities	959,377	898,876
U.S. government
Assets measured at Fair Value
Available-for-sale fixed income	281,707	335,223
U.S. Agency
Assets measured at Fair Value
Available-for-sale fixed income	26,194	37,927
Non-U.S. government & agency
Assets measured at Fair Value
Available-for-sale fixed income	16,471	13,547
Agency MBS
Assets measured at Fair Value
Available-for-sale fixed income	594,335	610,675
ABS/CMBS/MBS
Assets measured at Fair Value
Available-for-sale fixed income	758,032	672,984
Corporate
Assets measured at Fair Value
Available-for-sale fixed income	1,582,543	1,491,506
Municipal
Assets measured at Fair Value
Available-for-sale fixed income	358,660	371,474
Fair Value Measured on Recurring Basis
Assets measured at Fair Value
Available-for-sale fixed income	3,617,942	3,533,336
Equity securities	959,377	898,876
Total assets at fair value	4,577,319	4,432,212
Fair Value Measured on Recurring Basis \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets measured at Fair Value
Equity securities	950,058	890,622
Total assets at fair value	950,058	890,622
Fair Value Measured on Recurring Basis \| Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	3,482,677	3,423,029
Total assets at fair value	3,482,677	3,423,029
Fair Value Measured on Recurring Basis \| Significant Unobservable Inputs (Level 3)
Assets measured at Fair Value
Available-for-sale fixed income	135,265	110,307
Equity securities	9,319	8,254
Total assets at fair value	144,584	118,561
Fair Value Measured on Recurring Basis \| U.S. government
Assets measured at Fair Value
Available-for-sale fixed income	281,707	335,223
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	281,707	335,223
Fair Value Measured on Recurring Basis \| U.S. Agency
Assets measured at Fair Value
Available-for-sale fixed income	26,194	37,927
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	26,194	37,927
Fair Value Measured on Recurring Basis \| Non-U.S. government & agency
Assets measured at Fair Value
Available-for-sale fixed income	16,471	13,547
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	14,349	11,417
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	2,122	2,130
Fair Value Measured on Recurring Basis \| Agency MBS
Assets measured at Fair Value
Available-for-sale fixed income	594,335	610,675
Fair Value Measured on Recurring Basis \| Agency MBS \| Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	594,335	610,675
Fair Value Measured on Recurring Basis \| ABS/CMBS/MBS
Assets measured at Fair Value
Available-for-sale fixed income	758,032	672,984
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	758,032	672,984
Fair Value Measured on Recurring Basis \| Corporate
Assets measured at Fair Value
Available-for-sale fixed income	1,582,543	1,491,506
Fair Value Measured on Recurring Basis \| Corporate \| Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	1,449,400	1,383,329
Fair Value Measured on Recurring Basis \| Corporate \| Significant Unobservable Inputs (Level 3)
Assets measured at Fair Value
Available-for-sale fixed income	133,143	108,177
Fair Value Measured on Recurring Basis \| Municipal
Assets measured at Fair Value
Available-for-sale fixed income	358,660	371,474
Fair Value Measured on Recurring Basis \| Municipal \| Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	$ 358,660	$ 371,474

Fair value portion of asset recognized for present right to economic benefit.

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Amount excluding accrued interest, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI), classified as current.

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