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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 | Jun. 30, 2025 | Dec. 31, 2024
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Amortized Cost | $ 265,902 | $ 289,712
Debt Securities, Available-for-Sale | 264,055 |
Amortized Cost, Due in one year or less | 138,597 |
Amortized Cost, Due after one year through five years | 53,212 |
Amortized Cost, Due after five years through ten years | 62,642 |
Amortized Cost, Due after ten years | 978 |
Amortized Cost, Total | 255,429 |
Estimated Fair Value, Due in one year or less | 138,260 |
Estimated Fair Value, Due after one year through five years | 52,386 |
Estimated Fair Value, Due after five years through ten years | 61,934 |
Estimated Fair Value, Due after ten years | 980 |
Estimated Fair Value due, Total | 253,560 |
Fixed Income Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Amortized Cost | 163,183 | 162,124
Debt Securities, Available-for-Sale | 161,335 | 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 | 3,352 | 2,475
Debt Securities, Available-for-Sale | 3,362 | 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 | 1,935 |
Debt Securities, Available-for-Sale | 1,920 |
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Amortized Cost | 2,642 | 3,131
Debt Securities, Available-for-Sale | 2,652 | 3,142
Fixed Income Securities [Member] | Asset-backed Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Amortized Cost | 2,544 | 5,049
Debt Securities, Available-for-Sale | $ 2,561 | $ 5,061

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