Investments - Assets Measured at Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2025	Dec. 31, 2024
Assets measured at Fair Value
Available-for-sale fixed income	$ 3,420,761	$ 3,175,796
Equity securities	810,959	736,191
U.S. government
Assets measured at Fair Value
Available-for-sale fixed income	702,072	515,635
U.S. Agency
Assets measured at Fair Value
Available-for-sale fixed income	55,998	54,338
Non-U.S. government & agency
Assets measured at Fair Value
Available-for-sale fixed income	12,088	7,871
Agency MBS
Assets measured at Fair Value
Available-for-sale fixed income	398,548	396,223
ABS/CMBS/MBS
Assets measured at Fair Value
Available-for-sale fixed income	418,389	410,248
Corporate
Assets measured at Fair Value
Available-for-sale fixed income	1,405,927	1,346,521
Municipal
Assets measured at Fair Value
Available-for-sale fixed income	427,739	444,960
Fair Value Measured on Recurring Basis
Assets measured at Fair Value
Available-for-sale fixed income	3,420,761	3,175,796
Equity securities	810,959	736,191
Total assets at fair value	4,231,720	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	806,721	731,569
Total assets at fair value	806,721	731,569
Fair Value Measured on Recurring Basis \| Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	3,323,465	3,085,293
Total assets at fair value	3,323,465	3,085,293
Fair Value Measured on Recurring Basis \| Significant Unobservable Inputs (Level 3)
Assets measured at Fair Value
Available-for-sale fixed income	97,296	90,503
Equity securities	4,238	4,622
Total assets at fair value	101,534	95,125
Fair Value Measured on Recurring Basis \| U.S. government
Assets measured at Fair Value
Available-for-sale fixed income	702,072	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	702,072	515,635
Fair Value Measured on Recurring Basis \| U.S. Agency
Assets measured at Fair Value
Available-for-sale fixed income	55,998	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	55,998	54,338
Fair Value Measured on Recurring Basis \| Non-U.S. government & agency
Assets measured at Fair Value
Available-for-sale fixed income	12,088	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	11,089	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	999	973
Fair Value Measured on Recurring Basis \| Agency MBS
Assets measured at Fair Value
Available-for-sale fixed income	398,548	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	398,548	396,223
Fair Value Measured on Recurring Basis \| ABS/CMBS/MBS
Assets measured at Fair Value
Available-for-sale fixed income	418,389	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	418,389	410,248
Fair Value Measured on Recurring Basis \| Corporate
Assets measured at Fair Value
Available-for-sale fixed income	1,405,927	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	1,309,630	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	96,297	89,530
Fair Value Measured on Recurring Basis \| Municipal
Assets measured at Fair Value
Available-for-sale fixed income	427,739	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	$ 427,739	$ 444,960

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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