Investments - Assets Measured at Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2026	Dec. 31, 2025
Assets measured at Fair Value
Available-for-sale fixed income	$ 3,528,692	$ 3,533,336
Equity securities	864,912	898,876
U.S. government
Assets measured at Fair Value
Available-for-sale fixed income	307,140	335,223
U.S. Agency
Assets measured at Fair Value
Available-for-sale fixed income	27,288	37,927
Non-U.S. government & agency
Assets measured at Fair Value
Available-for-sale fixed income	16,286	13,547
Agency MBS
Assets measured at Fair Value
Available-for-sale fixed income	600,802	610,675
ABS/CMBS/MBS
Assets measured at Fair Value
Available-for-sale fixed income	710,301	672,984
Corporate
Assets measured at Fair Value
Available-for-sale fixed income	1,504,405	1,491,506
Municipal
Assets measured at Fair Value
Available-for-sale fixed income	362,470	371,474
Fair Value Measured on Recurring Basis
Assets measured at Fair Value
Available-for-sale fixed income	3,528,692	3,533,336
Equity securities	864,912	898,876
Total assets at fair value	4,393,604	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	856,689	890,622
Total assets at fair value	856,689	890,622
Fair Value Measured on Recurring Basis | Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	3,404,934	3,423,029
Total assets at fair value	3,404,934	3,423,029
Fair Value Measured on Recurring Basis | Significant Unobservable Inputs (Level 3)
Assets measured at Fair Value
Available-for-sale fixed income	123,758	110,307
Equity securities	8,223	8,254
Total assets at fair value	131,981	118,561
Fair Value Measured on Recurring Basis | U.S. government
Assets measured at Fair Value
Available-for-sale fixed income	307,140	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	307,140	335,223
Fair Value Measured on Recurring Basis | U.S. Agency
Assets measured at Fair Value
Available-for-sale fixed income	27,288	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	27,288	37,927
Fair Value Measured on Recurring Basis | Non-U.S. government & agency
Assets measured at Fair Value
Available-for-sale fixed income	16,286	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	14,191	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	2,095	2,130
Fair Value Measured on Recurring Basis | Agency MBS
Assets measured at Fair Value
Available-for-sale fixed income	600,802	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	600,802	610,675
Fair Value Measured on Recurring Basis | ABS/CMBS/MBS
Assets measured at Fair Value
Available-for-sale fixed income	710,301	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	710,301	672,984
Fair Value Measured on Recurring Basis | Corporate
Assets measured at Fair Value
Available-for-sale fixed income	1,504,405	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	1,382,742	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	121,663	108,177
Fair Value Measured on Recurring Basis | Municipal
Assets measured at Fair Value
Available-for-sale fixed income	362,470	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	$ 362,470	$ 371,474