Investments - Summary of Fixed Maturity Securities Classified as Available-for-Sale (Details) - USD ($) $ in Thousands	Mar. 31, 2026	Dec. 31, 2025
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 6,229,802	$ 6,081,997
Gross unrealized gains	53,596	97,471
Gross unrealized losses	(149,609)	(128,926)
Fair value	6,133,789	6,050,542
U.S. government, agencies and GSEs
Debt Securities, Available-for-sale [Line Items]
Amortized cost	268,060	253,706
Gross unrealized gains	2,941	4,893
Gross unrealized losses	(2,279)	(1,292)
Fair value	268,722	257,307
State and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized cost	521,242	527,584
Gross unrealized gains	1,633	1,914
Gross unrealized losses	(53,043)	(50,526)
Fair value	469,832	478,972
Non-U.S. government
Debt Securities, Available-for-sale [Line Items]
Amortized cost	179,045	180,669
Gross unrealized gains	2,178	5,176
Gross unrealized losses	(1,680)	(383)
Fair value	179,543	185,462
U.S. corporate
Debt Securities, Available-for-sale [Line Items]
Amortized cost	2,855,124	2,806,747
Gross unrealized gains	29,156	54,322
Gross unrealized losses	(60,284)	(50,342)
Fair value	2,823,996	2,810,727
Non-U.S. corporate
Debt Securities, Available-for-sale [Line Items]
Amortized cost	821,423	778,640
Gross unrealized gains	9,127	17,020
Gross unrealized losses	(14,226)	(12,604)
Fair value	816,324	783,056
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Amortized cost	338,304	342,915
Gross unrealized gains	3,889	6,496
Gross unrealized losses	(709)	(78)
Fair value	341,484	349,333
Commercial mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Amortized cost	199,514	128,692
Gross unrealized gains	866	1,355
Gross unrealized losses	(1,748)	(485)
Fair value	198,632	129,562
Other asset-backed
Debt Securities, Available-for-sale [Line Items]
Amortized cost	1,047,090	1,063,044
Gross unrealized gains	3,806	6,295
Gross unrealized losses	(15,640)	(13,216)
Fair value	$ 1,035,256	$ 1,056,123

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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