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Investments - Summary of Fixed Maturity Securities Classified as Available-for-Sale (Details) - USD ($) $ in Thousands	Mar. 31, 2025	Dec. 31, 2024
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 5,992,370	$ 5,876,502
Gross unrealized gains	37,418	20,711
Gross unrealized losses	(214,178)	(272,440)
Allowance for credit losses	273	0
Fair value	5,815,337	5,624,773
Total investments
Debt Securities, Available-for-sale [Line Items]
Amortized cost	5,996,062	5,879,872
Gross unrealized gains	37,422	20,712
Gross unrealized losses	(214,178)	(272,444)
Allowance for credit losses	273
Fair value	5,819,033	5,628,140
U.S. government, agencies and GSEs
Debt Securities, Available-for-sale [Line Items]
Amortized cost	287,643	281,858
Gross unrealized gains	2,800	590
Gross unrealized losses	(1,435)	(5,085)
Allowance for credit losses	0
Fair value	289,008	277,363
State and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized cost	540,495	541,715
Gross unrealized gains	1,571	1,388
Gross unrealized losses	(65,925)	(75,627)
Allowance for credit losses	0
Fair value	476,141	467,476
Non-U.S. government
Debt Securities, Available-for-sale [Line Items]
Amortized cost	113,342	85,566
Gross unrealized gains	780	94
Gross unrealized losses	(969)	(1,858)
Allowance for credit losses	0
Fair value	113,153	83,802
U.S. corporate
Debt Securities, Available-for-sale [Line Items]
Amortized cost	2,978,401	2,943,900
Gross unrealized gains	20,555	11,363
Gross unrealized losses	(98,751)	(129,584)
Allowance for credit losses	0
Fair value	2,900,205	2,825,679
Non-U.S. corporate
Debt Securities, Available-for-sale [Line Items]
Amortized cost	786,865	800,141
Gross unrealized gains	5,589	2,533
Gross unrealized losses	(23,228)	(30,050)
Allowance for credit losses	273
Fair value	768,953	772,624
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Amortized cost	111,660	8,394
Gross unrealized gains	505	27
Gross unrealized losses	(482)	(57)
Allowance for credit losses	0
Fair value	111,683	8,364
Other asset-backed
Debt Securities, Available-for-sale [Line Items]
Amortized cost	1,153,367	1,214,928
Gross unrealized gains	5,513	4,716
Gross unrealized losses	(23,388)	(30,179)
Allowance for credit losses	0
Fair value	1,135,492	1,189,465
Short-term investments
Debt Securities, Available-for-sale [Line Items]
Amortized cost	3,692	3,370
Gross unrealized gains	4	1
Gross unrealized losses	0	(4)
Allowance for credit losses	0
Fair value	3,696	$ 3,367
Commercial mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Amortized cost	20,597
Gross unrealized gains	105
Gross unrealized losses	0
Allowance for credit losses	0
Fair value	$ 20,702

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