Investments - Assets Measured at Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2025	Dec. 31, 2024
Assets measured at Fair Value
Available-for-sale fixed income	$ 3,274,276	$ 3,175,796
Equity securities	725,946	736,191
U.S. government
Assets measured at Fair Value
Available-for-sale fixed income	560,309	515,635
U.S. Agency
Assets measured at Fair Value
Available-for-sale fixed income	55,641	54,338
Non-U.S. government & agency
Assets measured at Fair Value
Available-for-sale fixed income	11,884	7,871
Agency MBS
Assets measured at Fair Value
Available-for-sale fixed income	403,391	396,223
ABS/CMBS/MBS
Assets measured at Fair Value
Available-for-sale fixed income	418,741	410,248
Corporate
Assets measured at Fair Value
Available-for-sale fixed income	1,384,635	1,346,521
Municipal
Assets measured at Fair Value
Available-for-sale fixed income	439,675	444,960
Fair Value Measured on Recurring Basis
Assets measured at Fair Value
Available-for-sale fixed income	3,274,276	3,175,796
Equity securities	725,946	736,191
Total assets at fair value	4,000,222	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	721,268	731,569
Total assets at fair value	721,268	731,569
Fair Value Measured on Recurring Basis | Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	3,180,009	3,085,293
Total assets at fair value	3,180,009	3,085,293
Fair Value Measured on Recurring Basis | Significant Unobservable Inputs (Level 3)
Assets measured at Fair Value
Available-for-sale fixed income	94,267	90,503
Equity securities	4,678	4,622
Total assets at fair value	98,945	95,125
Fair Value Measured on Recurring Basis | U.S. government
Assets measured at Fair Value
Available-for-sale fixed income	560,309	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	560,309	515,635
Fair Value Measured on Recurring Basis | U.S. Agency
Assets measured at Fair Value
Available-for-sale fixed income	55,641	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	55,641	54,338
Fair Value Measured on Recurring Basis | Non-U.S. government & agency
Assets measured at Fair Value
Available-for-sale fixed income	11,884	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	10,891	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	993	973
Fair Value Measured on Recurring Basis | Agency MBS
Assets measured at Fair Value
Available-for-sale fixed income	403,391	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	403,391	396,223
Fair Value Measured on Recurring Basis | ABS/CMBS/MBS
Assets measured at Fair Value
Available-for-sale fixed income	418,741	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	418,741	410,248
Fair Value Measured on Recurring Basis | Corporate
Assets measured at Fair Value
Available-for-sale fixed income	1,384,635	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	1,291,361	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	93,274	89,530
Fair Value Measured on Recurring Basis | Municipal
Assets measured at Fair Value
Available-for-sale fixed income	439,675	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	$ 439,675	$ 444,960