Fair Value Measurements - Summary of Carrying Amount and Fair Value of Ambac's Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2022	Dec. 31, 2021
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value	$ 1,977,000
Financial assets:
Derivative assets	29,000	$ 76,000
Other assets	0
Long-term Debt	2,201,000	2,230,000
Financial liabilities:
Derivative liabilities	40,000	95,000
Equity Securities Excluded From Fair Value Table	12,000	8,000
Short-term Investments [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value	523,000	414,000
Total Fair Value [Member]
Financial assets:
Cash	35,000	21,000
Reported Value Measurement [Member]
Financial assets:
Other investments	64,000	106,000
Cash	35,000	21,000
Loans	22,000	3,000
Total financial assets	6,589,000	9,268,000
Financial liabilities:
Liabilities for net financial guarantees written	(1,271,000)	(866,000)
Long-term debt	2,777,000	2,806,000
Total financial liabilities	5,407,000	8,190,000
Reported Value Measurement [Member] | Short-term Investments [Member]
Financial assets:
Short term investments	523,000	414,000
Reported Value Measurement [Member] | Other Debt Obligations [Member]
Financial assets:
Other investments	559,000	690,000
Reported Value Measurement [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities	40,000	94,000
Reported Value Measurement [Member] | Warrants [Member]
Financial assets:
Derivative assets	1,000
Other Assets [Member] | Reported Value Measurement [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	27,000	76,000
Level 1 [Member]
Financial assets:
Cash	35,000	21,000
Total financial assets	824,000	750,000
Financial liabilities:
Liabilities for net financial guarantees written	0	0
Long-term debt	0	0
Total financial liabilities	0	0
Level 1 [Member] | Short-term Investments [Member]
Financial assets:
Short term investments	521,000	369,000
Level 1 [Member] | Loans Receivable [Member]
Financial assets:
Loans	0	0
Level 1 [Member] | Other Debt Obligations [Member]
Financial assets:
Other investments	64,000	106,000
Level 1 [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities	0	0
Level 1 [Member] | Warrants [Member]
Financial assets:
Derivative assets	0
Level 1 [Member] | Other Assets [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	0	0
Level 2 [Member]
Financial assets:
Cash	0	1,000
Total financial assets	1,435,000	1,732,000
Financial liabilities:
Liabilities for net financial guarantees written	0	0
Long-term debt	2,171,000	2,575,000
Total financial liabilities	5,954,000	8,695,000
Level 2 [Member] | Short-term Investments [Member]
Financial assets:
Short term investments	2,000	46,000
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] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities	40,000	94,000
Level 2 [Member] | Warrants [Member]
Financial assets:
Derivative assets	0
Level 2 [Member] | Other Assets [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	2,000	5,000
Level 3 [Member]
Financial assets:
Cash	0	0
Total financial assets	3,834,000	6,202,000
Financial liabilities:
Liabilities for net financial guarantees written	(899,000)	(112,000)
Long-term debt	13,000	22,000
Total financial liabilities	(734,000)	79,000
Level 3 [Member] | Short-term Investments [Member]
Financial assets:
Short term investments	0	0
Level 3 [Member] | Loans Receivable [Member]
Financial assets:
Loans	22,000	3,000
Level 3 [Member] | Other Debt Obligations [Member]
Financial assets:
Other investments	0	0
Level 3 [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities	0	0
Level 3 [Member] | Warrants [Member]
Financial assets:
Derivative assets	1,000
Level 3 [Member] | Other Assets [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	25,000	71,000
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	160,000	340,000
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	658,000	613,000
Debt Security, Government, Non-US [Member] | Reported Value Measurement [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value	69,000	87,000
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	63,000	45,000
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	180,000	252,000
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	135,000	128,000
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	224,000	265,000
Fair Value, Recurring [Member]
Financial assets:
Total financial assets	6,577,000	9,261,000
Financial liabilities:
Liabilities for net financial guarantees written	(899,000)	(112,000)
Long-term debt	2,183,000	2,598,000
Total financial liabilities	5,221,000	8,775,000
Fair Value, Recurring [Member] | Short-term Investments [Member]
Financial assets:
Short term investments	523,000	414,000
Fair Value, Recurring [Member] | Loans Receivable [Member]
Financial assets:
Loans	22,000	3,000
Fair Value, Recurring [Member] | Other Debt Obligations [Member]
Financial assets:
Other investments	547,000	683,000
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	224,000	265,000
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	135,000	128,000
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	180,000	252,000
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	63,000	45,000
Fair Value, Recurring [Member] | Debt Security, Government, Non-US [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value	69,000	87,000
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	658,000	613,000
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	160,000	340,000
Fair Value, Recurring [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities	40,000	94,000
Fair Value, Recurring [Member] | Warrants [Member]
Financial assets:
Derivative assets	1,000
Fair Value, Recurring [Member] | Other Assets [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	27,000	76,000
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	63,000	45,000
Fair Value, Recurring [Member] | Level 1 [Member] | Debt Security, Government, Non-US [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value	69,000	87,000
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	1,000
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] | Asset-backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value	158,000	187,000
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	135,000	128,000
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	180,000	252,000
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] | Debt Security, Government, Non-US [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	646,000	600,000
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	160,000	340,000
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	66,000	79,000
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] | Debt Security, Government, Non-US [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	12,000	12,000
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
Collateral Pledged [Member] | 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	15,000	15,000
Collateral Pledged [Member] | Short-term Investments [Member] | Reported Value Measurement [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value	55,000	105,000
Collateral Pledged [Member] | Fair Value, Recurring [Member] | Short-term Investments [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value	55,000	105,000
Collateral Pledged [Member] | 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	15,000	15,000
Collateral Pledged [Member] | Fair Value, Recurring [Member] | Level 1 [Member] | Short-term Investments [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value	55,000	105,000
Collateral Pledged [Member] | 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	15,000	15,000
Collateral Pledged [Member] | Fair Value, Recurring [Member] | Level 2 [Member] | Short-term Investments [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value	0	0
Collateral Pledged [Member] | 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
Collateral Pledged [Member] | Fair Value, Recurring [Member] | Level 3 [Member] | Short-term Investments [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value	0	0
Collateral Pledged [Member] | 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
Variable Interest Entity, Primary Beneficiary [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value	2,119,000	3,455,000
Financial assets:
Loans	1,682,000	2,718,000
Derivative assets	61,000	38,000
Long-term Debt	2,752,000	4,216,000
Financial liabilities:
Derivative liabilities	1,111,000	1,940,000
Long-term debt	2,603,000	4,056,000
Variable Interest Entity, Primary Beneficiary [Member] | Corporate Debt Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value	2,026,000	3,320,000
Variable Interest Entity, Primary Beneficiary [Member] | Municipal Bonds [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value	92,000	136,000
Variable Interest Entity, Primary Beneficiary [Member] | Reported Value Measurement [Member]
Financial assets:
Restricted Cash and Cash Equivalents, Current	2,000	2,000
Loans	1,682,000	2,718,000
Financial liabilities:
Long-term debt	2,752,000	4,216,000
Variable Interest Entity, Primary Beneficiary [Member] | Reported Value Measurement [Member] | Corporate Debt Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value	2,026,000	3,320,000
Variable Interest Entity, Primary Beneficiary [Member] | Reported Value Measurement [Member] | Municipal Bonds [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value	92,000	136,000
Variable Interest Entity, Primary Beneficiary [Member] | Reported Value Measurement [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities	1,110,000	1,940,000
Variable Interest Entity, Primary Beneficiary [Member] | Reported Value Measurement [Member] | Currency Swaps [Member]
Financial assets:
Derivative assets	60,000	38,000
Variable Interest Entity, Primary Beneficiary [Member] | Level 1 [Member]
Financial assets:
Restricted Cash and Cash Equivalents, Current	2,000	2,000
Loans	0	0
Financial liabilities:
Long-term debt	0	0
Variable Interest Entity, Primary Beneficiary [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
Variable Interest Entity, Primary Beneficiary [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
Variable Interest Entity, Primary Beneficiary [Member] | Level 1 [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities	0	0
Variable Interest Entity, Primary Beneficiary [Member] | Level 1 [Member] | Currency Swaps [Member]
Financial assets:
Derivative assets	0	0
Variable Interest Entity, Primary Beneficiary [Member] | Level 2 [Member]
Financial assets:
Restricted Cash and Cash Equivalents, Current	0	0
Loans	0	0
Financial liabilities:
Long-term debt	2,634,000	4,086,000
Variable Interest Entity, Primary Beneficiary [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	0	0
Variable Interest Entity, Primary Beneficiary [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	92,000	136,000
Variable Interest Entity, Primary Beneficiary [Member] | Level 2 [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities	1,110,000	1,940,000
Variable Interest Entity, Primary Beneficiary [Member] | Level 2 [Member] | Currency Swaps [Member]
Financial assets:
Derivative assets	60,000	38,000
Variable Interest Entity, Primary Beneficiary [Member] | Level 3 [Member]
Financial assets:
Restricted Cash and Cash Equivalents, Current	0	0
Loans	1,682,000	2,718,000
Financial liabilities:
Long-term debt	153,000	169,000
Variable Interest Entity, Primary Beneficiary [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	2,026,000	3,320,000
Variable Interest Entity, Primary Beneficiary [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
Variable Interest Entity, Primary Beneficiary [Member] | Level 3 [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities	0	0
Variable Interest Entity, Primary Beneficiary [Member] | Level 3 [Member] | Currency Swaps [Member]
Financial assets:
Derivative assets	0	0
Variable Interest Entity, Primary Beneficiary [Member] | Fair Value, Recurring [Member]
Financial assets:
Restricted Cash and Cash Equivalents, Current	2,000	2,000
Loans	1,682,000	2,718,000
Financial liabilities:
Long-term debt	2,787,000	4,255,000
Variable Interest Entity, Primary Beneficiary [Member] | 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	2,026,000	3,320,000
Variable Interest Entity, Primary Beneficiary [Member] | 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	92,000	136,000
Variable Interest Entity, Primary Beneficiary [Member] | Fair Value, Recurring [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities	1,110,000	1,940,000
Variable Interest Entity, Primary Beneficiary [Member] | Fair Value, Recurring [Member] | Currency Swaps [Member]
Financial assets:
Derivative assets	$ 60,000	$ 38,000