Investments - Assets Measured at Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Assets measured at Fair Value
Available-for-sale fixed income	$ 2,989,527	$ 2,855,849
Equity securities	666,563	590,041
U.S. government
Assets measured at Fair Value
Available-for-sale fixed income	434,800	308,031
U.S. Agency
Assets measured at Fair Value
Available-for-sale fixed income	58,309	59,826
Non-U.S. government & agency
Assets measured at Fair Value
Available-for-sale fixed income	3,828	3,882
Agency MBS
Assets measured at Fair Value
Available-for-sale fixed income	414,675	425,285
ABS/CMBS/MBS
Assets measured at Fair Value
Available-for-sale fixed income	320,061	281,182
Corporate Debt
Assets measured at Fair Value
Available-for-sale fixed income	1,250,780	1,225,019
Municipal
Assets measured at Fair Value
Available-for-sale fixed income	507,074	552,624
Fair Value Measured on Recurring Basis
Assets measured at Fair Value
Available-for-sale fixed income	2,989,527	2,855,849
Equity securities	666,563	590,041
Total assets at fair value	3,656,090	3,445,890
Fair Value Measured on Recurring Basis \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets measured at Fair Value
Equity securities	661,978	588,416
Total assets at fair value	661,978	588,416
Fair Value Measured on Recurring Basis \| Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	2,921,432	2,795,378
Total assets at fair value	2,921,432	2,795,378
Fair Value Measured on Recurring Basis \| Significant Unobservable Inputs (Level 3)
Assets measured at Fair Value
Available-for-sale fixed income	68,095	60,471
Equity securities	4,585	1,625
Total assets at fair value	72,680	62,096
Fair Value Measured on Recurring Basis \| U.S. government
Assets measured at Fair Value
Available-for-sale fixed income	434,800	308,031
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	434,800	308,031
Fair Value Measured on Recurring Basis \| U.S. Agency
Assets measured at Fair Value
Available-for-sale fixed income	58,309	59,826
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	58,309	59,826
Fair Value Measured on Recurring Basis \| Non-U.S. government & agency
Assets measured at Fair Value
Available-for-sale fixed income	3,828	3,882
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	3,828	3,882
Fair Value Measured on Recurring Basis \| Agency MBS
Assets measured at Fair Value
Available-for-sale fixed income	414,675	425,285
Fair Value Measured on Recurring Basis \| Agency MBS \| Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	414,675	425,285
Fair Value Measured on Recurring Basis \| ABS/CMBS/MBS
Assets measured at Fair Value
Available-for-sale fixed income	320,061	281,182
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	320,061	281,182
Fair Value Measured on Recurring Basis \| Corporate Debt
Assets measured at Fair Value
Available-for-sale fixed income	1,250,780	1,225,019
Fair Value Measured on Recurring Basis \| Corporate Debt \| Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	1,182,685	1,164,548
Fair Value Measured on Recurring Basis \| Corporate Debt \| Significant Unobservable Inputs (Level 3)
Assets measured at Fair Value
Available-for-sale fixed income	68,095	60,471
Fair Value Measured on Recurring Basis \| Municipal
Assets measured at Fair Value
Available-for-sale fixed income	507,074	552,624
Fair Value Measured on Recurring Basis \| Municipal \| Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	$ 507,074	$ 552,624

Fair value portion of asset recognized for present right to economic benefit.

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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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