Investments - Assets Measured at Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Assets measured at Fair Value
Available-for-sale fixed income	$ 2,701,814	$ 2,666,950
Equity securities	519,097	498,382
U.S. government
Assets measured at Fair Value
Available-for-sale fixed income	385,199	454,021
U.S. Agency
Assets measured at Fair Value
Available-for-sale fixed income	73,659	73,063
Non-U.S. government & agency
Assets measured at Fair Value
Available-for-sale fixed income	5,952	5,847
Agency MBS
Assets measured at Fair Value
Available-for-sale fixed income	359,974	331,806
ABS/CMBS/MBS
Assets measured at Fair Value
Available-for-sale fixed income	246,382	240,736
Corporate Debt
Assets measured at Fair Value
Available-for-sale fixed income	1,083,500	1,034,330
Municipal
Assets measured at Fair Value
Available-for-sale fixed income	547,148	527,147
Fair Value Measured on Recurring Basis
Assets measured at Fair Value
Available-for-sale fixed income	2,701,814	2,666,950
Equity securities	519,097	498,382
Total assets at fair value	3,220,911	3,165,332
Fair Value Measured on Recurring Basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets measured at Fair Value
Equity securities	517,489	496,731
Total assets at fair value	517,489	496,731
Fair Value Measured on Recurring Basis | Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	2,649,053	2,613,296
Equity securities		39
Total assets at fair value	2,649,053	2,613,335
Fair Value Measured on Recurring Basis | Significant Unobservable Inputs (Level 3)
Assets measured at Fair Value
Available-for-sale fixed income	52,761	53,654
Equity securities	1,608	1,612
Total assets at fair value	54,369	55,266
Fair Value Measured on Recurring Basis | U.S. government
Assets measured at Fair Value
Available-for-sale fixed income	385,199	454,021
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	385,199	454,021
Fair Value Measured on Recurring Basis | U.S. Agency
Assets measured at Fair Value
Available-for-sale fixed income	73,659	73,063
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	73,659	73,063
Fair Value Measured on Recurring Basis | Non-U.S. government & agency
Assets measured at Fair Value
Available-for-sale fixed income	5,952	5,847
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	5,952	5,847
Fair Value Measured on Recurring Basis | Agency MBS
Assets measured at Fair Value
Available-for-sale fixed income	359,974	331,806
Fair Value Measured on Recurring Basis | Agency MBS | Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	359,974	331,806
Fair Value Measured on Recurring Basis | ABS/CMBS/MBS
Assets measured at Fair Value
Available-for-sale fixed income	246,382	240,736
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	246,382	240,736
Fair Value Measured on Recurring Basis | Corporate Debt
Assets measured at Fair Value
Available-for-sale fixed income	1,083,500	1,034,330
Fair Value Measured on Recurring Basis | Corporate Debt | Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	1,030,739	980,676
Fair Value Measured on Recurring Basis | Corporate Debt | Significant Unobservable Inputs (Level 3)
Assets measured at Fair Value
Available-for-sale fixed income	52,761	53,654
Fair Value Measured on Recurring Basis | Municipal
Assets measured at Fair Value
Available-for-sale fixed income	547,148	527,147
Fair Value Measured on Recurring Basis | Municipal | Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	$ 547,148	$ 527,147