Fair Value Measurements - Summary of Carrying Amount and Fair Value of Ambac's Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2025	Mar. 31, 2025	Dec. 31, 2024	Mar. 31, 2024
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value		$ 263,179
Debt Securities, Available-for-sale, Allowance for Credit Loss	$ 0	0	$ 0	$ 0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	260	881
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	5,351	3,994	5,351
Financial assets:
Derivative assets		210	0
Other assets		0
Short-Term Debt		150,000	150,000
Financial liabilities:
Derivative liabilities		0	317
Equity Securities Excluded From Fair Value Table		20,618	20,618
Debt Securities, Available-for-sale, Amortized Cost	289,712	266,292
Debt Securities, Available-for-Sale, Current	284,621	263,179
Equity Method Investments Excluded From Fair Value		176	177
Fixed Income Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value	157,020	161,569
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	247	875
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	5,351	3,994
Financial liabilities:
Debt Securities, Available-for-sale, Amortized Cost	162,124	164,688
Commercial Mortgage Backed Securities [Member] | Fixed Income Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value		2,049
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		22	34
Financial liabilities:
Debt Securities, Available-for-sale, Amortized Cost		2,067
Short-term Investments [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value	127,601	101,610
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	13	6
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0	0
Financial liabilities:
Debt Securities, Available-for-sale, Amortized Cost	127,588	101,604	127,588
Asset-backed Securities [Member] | Fixed Income Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value	5,061	4,787
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	14	16
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	2	2	2
Financial liabilities:
Debt Securities, Available-for-sale, Amortized Cost	5,049	4,773
Collateralized Debt Obligations [Member] | Fixed Income Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value	3,142	2,954
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	13	14
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	2	1	2
Financial liabilities:
Debt Securities, Available-for-sale, Amortized Cost	3,131	2,941
Residential Mortgage-Backed Securities [Member] | Fixed Income Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value	2,446	3,445
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	0	16
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	29	18	29
Financial liabilities:
Debt Securities, Available-for-sale, Amortized Cost	2,475	3,447
US Government Debt Securities [Member] | Fixed Income Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value	40,995	44,860
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	98	429
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	809	390	809
Financial liabilities:
Debt Securities, Available-for-sale, Amortized Cost	41,706	44,821
Debt Security, Government, Non-US [Member] | Fixed Income Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		0	0
Corporate Debt Securities [Member] | Fixed Income Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value	89,192	89,169
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	107	343
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	3,905	3,153	3,905
Financial liabilities:
Debt Securities, Available-for-sale, Amortized Cost	92,990	91,979
Municipal Bonds [Member] | Fixed Income Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value	14,083	14,305
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	7	53
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	570	408	570
Financial liabilities:
Debt Securities, Available-for-sale, Amortized Cost	14,646	14,660
Collateralized Mortgage Backed Securities [Member] | Fixed Income Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value	2,101	2,049
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	8	4
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	34	22
Financial liabilities:
Debt Securities, Available-for-sale, Amortized Cost	$ 2,127	2,067
Total Fair Value [Member]
Financial assets:
Cash		51,660	47,276
Reported Value Measurement [Member]
Financial assets:
Cash		51,660	47,276
Loans		3,406	3,434
Total financial assets		346,669	363,625
Short-Term Debt		152,695	152,560
Financial liabilities:
Total financial liabilities		152,695	152,877
Reported Value Measurement [Member] | Short-term Investments [Member]
Financial assets:
Short term investments		101,610	127,601
Reported Value Measurement [Member] | Other Debt Obligations [Member]
Financial assets:
Other investments		28,214	28,294
Reported Value Measurement [Member] | Warrants [Member]
Financial assets:
Derivative assets		0	0
Reported Value Measurement [Member] | Foreign Exchange Future
Financial liabilities:
Derivative liabilities		0	317
Reported Value Measurement [Member] | Foreign Exchange Forward
Financial assets:
Derivative assets		210	0
Level 1 [Member]
Financial assets:
Cash		51,660	47,276
Total financial assets		198,130	215,872
Short-Term Debt		0	0
Financial liabilities:
Total financial liabilities		0	0
Level 1 [Member] | Short-term Investments [Member]
Financial assets:
Short term investments		101,610	127,601
Level 1 [Member] | Loans Receivable [Member]
Financial assets:
Loans		0	0
Level 1 [Member] | Other Debt Obligations [Member]
Financial assets:
Other investments		0	0
Level 1 [Member] | Warrants [Member]
Financial assets:
Derivative assets		0	0
Level 1 [Member] | Foreign Exchange Future
Financial liabilities:
Derivative liabilities		0	0
Level 1 [Member] | Foreign Exchange Forward
Financial assets:
Derivative assets		0	0
Level 2 [Member]
Financial assets:
Cash		0	0
Total financial assets		116,919	116,025
Short-Term Debt		0	0
Financial liabilities:
Total financial liabilities		0	317
Level 2 [Member] | Short-term Investments [Member]
Financial assets:
Short term investments		0	0
Level 2 [Member] | Loans Receivable [Member]
Financial assets:
Loans		0	0
Level 2 [Member] | Other Debt Obligations [Member]
Financial assets:
Other investments		0	0
Level 2 [Member] | Warrants [Member]
Financial assets:
Derivative assets		0	0
Level 2 [Member] | Foreign Exchange Future
Financial liabilities:
Derivative liabilities		0	317
Level 2 [Member] | Foreign Exchange Forward
Financial assets:
Derivative assets		210	0
Level 3 [Member]
Financial assets:
Cash		0	0
Total financial assets		3,406	3,434
Short-Term Debt		152,695	152,560
Financial liabilities:
Total financial liabilities		152,695	152,560
Level 3 [Member] | Short-term Investments [Member]
Financial assets:
Short term investments		0	0
Level 3 [Member] | Loans Receivable [Member]
Financial assets:
Loans		3,406	3,434
Level 3 [Member] | Other Debt Obligations [Member]
Financial assets:
Other investments		0	0
Level 3 [Member] | Warrants [Member]
Financial assets:
Derivative assets		0	0
Level 3 [Member] | Foreign Exchange Future
Financial liabilities:
Derivative liabilities		0	0
Level 3 [Member] | Foreign Exchange Forward
Financial assets:
Derivative assets		0	0
Commercial Mortgage Backed Securities [Member] | Reported Value Measurement [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value		2,049	2,101
Municipal Bonds [Member] | Reported Value Measurement [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value		14,305	14,083
Corporate Debt Securities [Member] | Reported Value Measurement [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value		89,169	89,192
US Government Debt Securities [Member] | Reported Value Measurement [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value		44,860	40,995
Residential Mortgage-Backed Securities [Member] | Reported Value Measurement [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value		3,445	2,446
Collateralized Debt Obligations [Member] | Reported Value Measurement [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value		2,954	3,142
Asset-backed Securities [Member] | Reported Value Measurement [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value		4,787	5,061
Fair Value, Recurring [Member]
Financial assets:
Total financial assets		325,875	342,830
Short-Term Debt		152,695	152,560
Financial liabilities:
Total financial liabilities		152,695	152,877
Fair Value, Recurring [Member] | Commercial Mortgage Backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value		2,049	2,101
Fair Value, Recurring [Member] | Short-term Investments [Member]
Financial assets:
Short term investments		101,610	127,601
Fair Value, Recurring [Member] | Loans Receivable [Member]
Financial assets:
Loans		3,406	3,434
Fair Value, Recurring [Member] | Other Debt Obligations [Member]
Financial assets:
Other investments		7,420	7,499
Fair Value, Recurring [Member] | Asset-backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value		4,787	5,061
Fair Value, Recurring [Member] | Collateralized Debt Obligations [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value		2,954	3,142
Fair Value, Recurring [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value		3,445	2,446
Fair Value, Recurring [Member] | US Government Debt Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value		44,860	40,995
Fair Value, Recurring [Member] | Corporate Debt Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value		89,169	89,192
Fair Value, Recurring [Member] | Municipal Bonds [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value		14,305	14,083
Fair Value, Recurring [Member] | Warrants [Member]
Financial assets:
Derivative assets		0	0
Fair Value, Recurring [Member] | Foreign Exchange Future
Financial liabilities:
Derivative liabilities		0	317
Fair Value, Recurring [Member] | Foreign Exchange Forward
Financial assets:
Derivative assets		210	0
Fair Value, Recurring [Member] | Level 1 [Member] | Commercial Mortgage Backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value		0	0
Fair Value, Recurring [Member] | Level 1 [Member] | Asset-backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value		0	0
Fair Value, Recurring [Member] | Level 1 [Member] | Collateralized Debt Obligations [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value		0	0
Fair Value, Recurring [Member] | Level 1 [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value		0	0
Fair Value, Recurring [Member] | Level 1 [Member] | US Government Debt Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value		44,860	40,995
Fair Value, Recurring [Member] | Level 1 [Member] | Corporate Debt Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value		0	0
Fair Value, Recurring [Member] | Level 1 [Member] | Municipal Bonds [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value		0	0
Fair Value, Recurring [Member] | Level 2 [Member] | Commercial Mortgage Backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value		2,049	2,101
Fair Value, Recurring [Member] | Level 2 [Member] | Asset-backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value		4,787	5,061
Fair Value, Recurring [Member] | Level 2 [Member] | Collateralized Debt Obligations [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value		2,954	3,142
Fair Value, Recurring [Member] | Level 2 [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value		3,445	2,446
Fair Value, Recurring [Member] | Level 2 [Member] | US Government Debt Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value		0	0
Fair Value, Recurring [Member] | Level 2 [Member] | Corporate Debt Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value		89,169	89,192
Fair Value, Recurring [Member] | Level 2 [Member] | Municipal Bonds [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value		14,305	14,083
Fair Value, Recurring [Member] | Level 3 [Member] | Commercial Mortgage Backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value		0	0
Fair Value, Recurring [Member] | Level 3 [Member] | Asset-backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value		0	0
Fair Value, Recurring [Member] | Level 3 [Member] | Collateralized Debt Obligations [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value		0	0
Fair Value, Recurring [Member] | Level 3 [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value		0	0
Fair Value, Recurring [Member] | Level 3 [Member] | US Government Debt Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value		0	0
Fair Value, Recurring [Member] | Level 3 [Member] | Corporate Debt Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value		0	0
Fair Value, Recurring [Member] | Level 3 [Member] | Municipal Bonds [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value		$ 0	$ 0