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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 | Mar. 31, 2026 | Dec. 31, 2025
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Amortized Cost | $ 231,537 | $ 269,848
Debt Securities, Available-for-Sale | 229,387 | 268,737
Amortized Cost, Due in one year or less | 117,446 | 180,013
Amortized Cost, Due after one year through five years | 47,546 | 32,409
Amortized Cost, Due after five years through ten years | 55,344 | 47,801
Amortized Cost, Due after ten years | 550 | 979
Amortized Cost, Total | 220,886 | 261,202
Estimated Fair Value, Due in one year or less | 117,313 | 179,824
Estimated Fair Value, Due after one year through five years | 45,845 | 31,588
Estimated Fair Value, Due after five years through ten years | 55,042 | 47,619
Estimated Fair Value, Due after ten years | 553 | 998
Estimated Fair Value due, Total | 218,753 | 260,029
Fixed Income Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Amortized Cost | 139,242 | 123,414
Debt Securities, Available-for-Sale | 137,092 | 122,295
Fixed Income Securities [Member] | Residential Mortgage-Backed Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Amortized Cost | 2,643 | 1,604
Debt Securities, Available-for-Sale | 2,627 | 1,597
Fixed Income Securities [Member] | Commercial Mortgage Backed Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Amortized Cost | 5,002 | 3,307
Debt Securities, Available-for-Sale | 4,985 | 3,341
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Amortized Cost | 1,626 | 1,963
Debt Securities, Available-for-Sale | 1,630 | 1,975
Fixed Income Securities [Member] | Asset-backed Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Amortized Cost | 1,380 | 1,772
Debt Securities, Available-for-Sale | $ 1,392 | $ 1,795

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