Investments - Assets Measured at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023
Assets measured at Fair Value
Available-for-sale fixed income	$ 3,176,073	$ 2,855,849
Equity securities	729,738	590,041
U.S. government
Assets measured at Fair Value
Available-for-sale fixed income	480,207	308,031
U.S. Agency
Assets measured at Fair Value
Available-for-sale fixed income	69,111	59,826
Non-U.S. government & agency
Assets measured at Fair Value
Available-for-sale fixed income	7,261	3,882
Agency MBS
Assets measured at Fair Value
Available-for-sale fixed income	420,390	425,285
ABS/CMBS/MBS
Assets measured at Fair Value
Available-for-sale fixed income	361,625	281,182
Corporate
Assets measured at Fair Value
Available-for-sale fixed income	1,352,693	1,225,019
Municipal
Assets measured at Fair Value
Available-for-sale fixed income	484,786	552,624
Fair Value Measured on Recurring Basis
Assets measured at Fair Value
Available-for-sale fixed income	3,176,073	2,855,849
Equity securities	729,738	590,041
Total assets at fair value	3,905,811	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	725,047	588,416
Total assets at fair value	725,047	588,416
Fair Value Measured on Recurring Basis \| Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	3,089,237	2,795,378
Total assets at fair value	3,089,237	2,795,378
Fair Value Measured on Recurring Basis \| Significant Unobservable Inputs (Level 3)
Assets measured at Fair Value
Available-for-sale fixed income	86,836	60,471
Equity securities	4,691	1,625
Total assets at fair value	91,527	62,096
Fair Value Measured on Recurring Basis \| U.S. government
Assets measured at Fair Value
Available-for-sale fixed income	480,207	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	480,207	308,031
Fair Value Measured on Recurring Basis \| U.S. Agency
Assets measured at Fair Value
Available-for-sale fixed income	69,111	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	69,111	59,826
Fair Value Measured on Recurring Basis \| Non-U.S. government & agency
Assets measured at Fair Value
Available-for-sale fixed income	7,261	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	7,261	3,882
Fair Value Measured on Recurring Basis \| Agency MBS
Assets measured at Fair Value
Available-for-sale fixed income	420,390	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	420,390	425,285
Fair Value Measured on Recurring Basis \| ABS/CMBS/MBS
Assets measured at Fair Value
Available-for-sale fixed income	361,625	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	361,625	281,182
Fair Value Measured on Recurring Basis \| Corporate
Assets measured at Fair Value
Available-for-sale fixed income	1,352,693	1,225,019
Fair Value Measured on Recurring Basis \| Corporate \| Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	1,265,857	1,164,548
Fair Value Measured on Recurring Basis \| Corporate \| Significant Unobservable Inputs (Level 3)
Assets measured at Fair Value
Available-for-sale fixed income	86,836	60,471
Fair Value Measured on Recurring Basis \| Municipal
Assets measured at Fair Value
Available-for-sale fixed income	484,786	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	$ 484,786	$ 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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