Investments - Assets Measured at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Assets measured at Fair Value
Available-for-sale fixed income	$ 2,717,627	$ 2,666,950
Equity securities	534,436	498,382
U.S. government
Assets measured at Fair Value
Available-for-sale fixed income	321,611	454,021
U.S. Agency
Assets measured at Fair Value
Available-for-sale fixed income	48,368	73,063
Non-U.S. government & agency
Assets measured at Fair Value
Available-for-sale fixed income	3,811	5,847
Agency MBS
Assets measured at Fair Value
Available-for-sale fixed income	399,381	331,806
ABS/CMBS/MBS
Assets measured at Fair Value
Available-for-sale fixed income	265,779	240,736
Corporate Debt
Assets measured at Fair Value
Available-for-sale fixed income	1,153,384	1,034,330
Municipal
Assets measured at Fair Value
Available-for-sale fixed income	525,293	527,147
Fair Value Measured on Recurring Basis
Assets measured at Fair Value
Available-for-sale fixed income	2,717,627	2,666,950
Equity securities	534,436	498,382
Total assets at fair value	3,252,063	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	532,856	496,731
Total assets at fair value	532,856	496,731
Fair Value Measured on Recurring Basis \| Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	2,661,336	2,613,296
Equity securities		39
Total assets at fair value	2,661,336	2,613,335
Fair Value Measured on Recurring Basis \| Significant Unobservable Inputs (Level 3)
Assets measured at Fair Value
Available-for-sale fixed income	56,291	53,654
Equity securities	1,580	1,612
Total assets at fair value	57,871	55,266
Fair Value Measured on Recurring Basis \| U.S. government
Assets measured at Fair Value
Available-for-sale fixed income	321,611	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	321,611	454,021
Fair Value Measured on Recurring Basis \| U.S. Agency
Assets measured at Fair Value
Available-for-sale fixed income	48,368	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	48,368	73,063
Fair Value Measured on Recurring Basis \| Non-U.S. government & agency
Assets measured at Fair Value
Available-for-sale fixed income	3,811	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	3,811	5,847
Fair Value Measured on Recurring Basis \| Agency MBS
Assets measured at Fair Value
Available-for-sale fixed income	399,381	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	399,381	331,806
Fair Value Measured on Recurring Basis \| ABS/CMBS/MBS
Assets measured at Fair Value
Available-for-sale fixed income	265,779	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	265,779	240,736
Fair Value Measured on Recurring Basis \| Corporate Debt
Assets measured at Fair Value
Available-for-sale fixed income	1,153,384	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	1,097,093	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	56,291	53,654
Fair Value Measured on Recurring Basis \| Municipal
Assets measured at Fair Value
Available-for-sale fixed income	525,293	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	$ 525,293	$ 527,147

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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