Investments - Assets Measured at Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021
Assets measured at Fair Value
Available-for-sale fixed income	$ 2,666,950	$ 2,409,887
Equity securities	498,382	613,776
U.S. government
Assets measured at Fair Value
Available-for-sale fixed income	454,021	134,554
U.S. Agency
Assets measured at Fair Value
Available-for-sale fixed income	73,063	32,760
Non-U.S. government & agency
Assets measured at Fair Value
Available-for-sale fixed income	5,847	8,481
Agency MBS
Assets measured at Fair Value
Available-for-sale fixed income	331,806	367,187
ABS/CMBS/MBS
Assets measured at Fair Value
Available-for-sale fixed income	240,736	264,054
Corporate Debt
Assets measured at Fair Value
Available-for-sale fixed income	1,034,330	957,095
Municipal
Assets measured at Fair Value
Available-for-sale fixed income	527,147	645,756
Fair Value Measured on Recurring Basis
Assets measured at Fair Value
Available-for-sale fixed income	2,666,950	2,409,887
Equity securities	498,382	613,776
Total assets at fair value	3,165,332	3,023,663
Fair Value Measured on Recurring Basis \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets measured at Fair Value
Equity securities	496,731	613,712
Total assets at fair value	496,731	613,712
Fair Value Measured on Recurring Basis \| Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	2,613,296	2,366,369
Equity securities	39	64
Total assets at fair value	2,613,335	2,366,433
Fair Value Measured on Recurring Basis \| Significant Unobservable Inputs (Level 3)
Assets measured at Fair Value
Available-for-sale fixed income	53,654	43,518
Equity securities	1,612
Total assets at fair value	55,266	43,518
Fair Value Measured on Recurring Basis \| U.S. government
Assets measured at Fair Value
Available-for-sale fixed income	454,021	134,554
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	454,021	134,554
Fair Value Measured on Recurring Basis \| U.S. Agency
Assets measured at Fair Value
Available-for-sale fixed income	73,063	32,760
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	73,063	32,760
Fair Value Measured on Recurring Basis \| Non-U.S. government & agency
Assets measured at Fair Value
Available-for-sale fixed income	5,847	8,481
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	5,847	8,481
Fair Value Measured on Recurring Basis \| Agency MBS
Assets measured at Fair Value
Available-for-sale fixed income	331,806	367,187
Fair Value Measured on Recurring Basis \| Agency MBS \| Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	331,806	367,187
Fair Value Measured on Recurring Basis \| ABS/CMBS/MBS
Assets measured at Fair Value
Available-for-sale fixed income	240,736	264,054
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	240,736	264,054
Fair Value Measured on Recurring Basis \| Corporate Debt
Assets measured at Fair Value
Available-for-sale fixed income	1,034,330	957,095
Fair Value Measured on Recurring Basis \| Corporate Debt \| Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	980,676	913,577
Fair Value Measured on Recurring Basis \| Corporate Debt \| Significant Unobservable Inputs (Level 3)
Assets measured at Fair Value
Available-for-sale fixed income	53,654	43,518
Fair Value Measured on Recurring Basis \| Municipal
Assets measured at Fair Value
Available-for-sale fixed income	527,147	645,756
Fair Value Measured on Recurring Basis \| Municipal \| Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	$ 527,147	$ 645,756

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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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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