Investments - Summary of Fixed Maturity Securities Classified as Available-for-Sale (Details) - USD ($) $ in Thousands	Jun. 30, 2026	Dec. 31, 2025
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 6,258,735	$ 6,081,997
Gross unrealized gains	47,979	97,471
Gross unrealized losses	(144,739)	(128,926)
Fair value	6,161,975	6,050,542
Total investments
Debt Securities, Available-for-sale [Line Items]
Amortized cost	6,307,881
Gross unrealized gains	47,979
Gross unrealized losses	(144,762)
Fair value	6,211,098
U.S. government, agencies and GSEs
Debt Securities, Available-for-sale [Line Items]
Amortized cost	286,696	253,706
Gross unrealized gains	2,398	4,893
Gross unrealized losses	(2,952)	(1,292)
Fair value	286,142	257,307
State and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized cost	518,496	527,584
Gross unrealized gains	1,493	1,914
Gross unrealized losses	(53,537)	(50,526)
Fair value	466,452	478,972
Non-U.S. government
Debt Securities, Available-for-sale [Line Items]
Amortized cost	196,938	180,669
Gross unrealized gains	3,009	5,176
Gross unrealized losses	(1,098)	(383)
Fair value	198,849	185,462
U.S. corporate
Debt Securities, Available-for-sale [Line Items]
Amortized cost	2,805,763	2,806,747
Gross unrealized gains	26,080	54,322
Gross unrealized losses	(54,969)	(50,342)
Fair value	2,776,874	2,810,727
Non-U.S. corporate
Debt Securities, Available-for-sale [Line Items]
Amortized cost	854,287	778,640
Gross unrealized gains	8,758	17,020
Gross unrealized losses	(13,054)	(12,604)
Fair value	849,991	783,056
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Amortized cost	356,566	342,915
Gross unrealized gains	2,490	6,496
Gross unrealized losses	(1,054)	(78)
Fair value	358,002	349,333
Commercial mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Amortized cost	225,858	128,692
Gross unrealized gains	826	1,355
Gross unrealized losses	(3,216)	(485)
Fair value	223,468	129,562
Other asset-backed
Debt Securities, Available-for-sale [Line Items]
Amortized cost	1,014,131	1,063,044
Gross unrealized gains	2,925	6,295
Gross unrealized losses	(14,859)	(13,216)
Fair value	1,002,197	$ 1,056,123
Short-term investments
Debt Securities, Available-for-sale [Line Items]
Amortized cost	49,146
Gross unrealized gains	0
Gross unrealized losses	(23)
Fair value	$ 49,123

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on 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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