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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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