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Investments - Summary of Amortized Cost and Estimated Fair Value of Available-for-Sale Investments, Excluding VIE Investments Held by Successor Ambac, by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	$ 428,432	$ 289,712
Debt Securities, Available-for-Sale	426,926
Amortized Cost, Due in one year or less	331,403
Amortized Cost, Due after one year through five years	35,584
Amortized Cost, Due after five years through ten years	50,797
Amortized Cost, Due after ten years	978
Amortized Cost, Total	418,762
Estimated Fair Value, Due in one year or less	331,164
Estimated Fair Value, Due after one year through five years	34,749
Estimated Fair Value, Due after five years through ten years	50,296
Estimated Fair Value, Due after ten years	987
Estimated Fair Value due, Total	417,196
Fixed Income Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	132,617	162,124
Debt Securities, Available-for-Sale	131,101	157,020
Fixed Income Securities [Member] | Residential Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	3,265	2,475
Debt Securities, Available-for-Sale	3,294	2,446
Fixed Income Securities [Member] | Commercial Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	1,827
Debt Securities, Available-for-Sale	1,819
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	2,304	3,131
Debt Securities, Available-for-Sale	2,319	3,142
Fixed Income Securities [Member] | Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	2,274	5,049
Debt Securities, Available-for-Sale	$ 2,298	$ 5,061

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