Fair Value Measurements - Summary of Carrying Amount and Fair Value of Ambac's Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value	$ 2,789,000
Financial assets:
Derivative assets	82,000	$ 93,000
Other assets	114	1,000
Long-term Debt	2,671,000	2,739,000
Financial liabilities:
Derivative liabilities	99,000	114,000
Equity Securities Excluded From Fair Value Table	8,000	0
Short-term Investments [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed maturity securities, at fair value	416,000	492,000
Total Fair Value [Member]
Financial assets:
Cash	30,000	33,000
Reported Value Measurement [Member]
Financial assets:
Other investments	110,000	89,000
Cash	30,000	33,000
Loans	3,000	3,000
Other assets	0	1,000
Total financial assets	9,858,000	10,071,000
Financial liabilities:
Liabilities for net financial guarantees written	(781,000)	(740,000)
Long-term debt	3,203,000	3,255,000
Total financial liabilities	8,754,000	8,958,000
Reported Value Measurement [Member] | Short-term Investments [Member]
Financial assets:
Short term investments	416,000	492,000
Reported Value Measurement [Member] | Other Debt Obligations [Member]
Financial assets:
Other investments	657,000	595,000
Reported Value Measurement [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	(1,000)
Financial liabilities:
Derivative liabilities	98,000	114,000
Other Assets [Member] | Reported Value Measurement [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	82,000	93,000
Derivative Asset, Fair Value, Gross Liability	(1,000)
Level 1 [Member]
Financial assets:
Cash	24,000	32,000
Total financial assets	720,000	888,000
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	329,000	415,000
Level 1 [Member] | Equity Method Investments [Member]
Financial assets:
Other assets	0	0
Level 1 [Member] | Loans Receivable [Member]
Financial assets:
Loans	0	0
Level 1 [Member] | Other Debt Obligations [Member]
Financial assets:
Other investments	110,000	91,000
Level 1 [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	0
Financial liabilities:
Derivative liabilities	0	0
Level 1 [Member] | Other Assets [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	0	0
Derivative Asset, Fair Value, Gross Liability	0
Level 2 [Member]
Financial assets:
Cash	7,000	2,000
Total financial assets	2,314,000	2,299,000
Financial liabilities:
Liabilities for net financial guarantees written	0	0
Long-term debt	3,018,000	2,670,000
Total financial liabilities	9,215,000	8,968,000
Level 2 [Member] | Short-term Investments [Member]
Financial assets:
Short term investments	87,000	76,000
Level 2 [Member] | Equity Method Investments [Member]
Financial assets:
Other assets	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] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	(1,000)
Financial liabilities:
Derivative liabilities	98,000	114,000
Level 2 [Member] | Other Assets [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	7,000	9,000
Derivative Asset, Fair Value, Gross Liability	(1,000)
Level 3 [Member]
Financial assets:
Cash	0	0
Total financial assets	6,276,000	6,433,000
Financial liabilities:
Liabilities for net financial guarantees written	186,000	539,000
Long-term debt	22,000	401,000
Total financial liabilities	382,000	1,095,000
Level 3 [Member] | Short-term Investments [Member]
Financial assets:
Short term investments	0	0
Level 3 [Member] | Equity Method Investments [Member]
Financial assets:
Other assets	0	1,000
Level 3 [Member] | Loans Receivable [Member]
Financial assets:
Loans	3,000	3,000
Level 3 [Member] | Other Debt Obligations [Member]
Financial assets:
Other investments	0	53,000
Level 3 [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	0
Financial liabilities:
Derivative liabilities	0	0
Level 3 [Member] | Other Assets [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	75,000	85,000
Derivative Asset, Fair Value, Gross Liability	0
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	358,000	358,000
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	1,106,000	1,077,000
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	89,000	98,000
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	43,000	106,000
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	245,000	302,000
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	111,000	74,000
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	301,000	303,000
Fair Value, Recurring [Member]
Financial assets:
Total financial assets	9,851,000	10,073,000
Financial liabilities:
Liabilities for net financial guarantees written	186,000	539,000
Long-term debt	3,040,000	3,071,000
Total financial liabilities	9,597,000	10,063,000
Fair Value, Recurring [Member] | Short-term Investments [Member]
Financial assets:
Short term investments	416,000	492,000
Fair Value, Recurring [Member] | Equity Method Investments [Member]
Financial assets:
Other assets	0	1,000
Fair Value, Recurring [Member] | Loans Receivable [Member]
Financial assets:
Loans	3,000	3,000
Fair Value, Recurring [Member] | Other Debt Obligations [Member]
Financial assets:
Other investments	650,000	597,000
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	301,000	303,000
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	111,000	74,000
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	245,000	302,000
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	43,000	106,000
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	89,000	98,000
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	1,106,000	1,077,000
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	358,000	358,000
Fair Value, Recurring [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	(1,000)
Financial liabilities:
Derivative liabilities	98,000	114,000
Fair Value, Recurring [Member] | Other Assets [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	82,000	93,000
Derivative Asset, Fair Value, Gross Liability	(1,000)
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	43,000	106,000
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	89,000	98,000
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	3,000	4,000
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	221,000	225,000
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	111,000	74,000
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	245,000	302,000
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	1,103,000	1,073,000
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	358,000	358,000
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	80,000	78,000
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	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
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
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	105,000	125,000
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	105,000	125,000
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
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	105,000	125,000
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
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
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
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	3,315,000	3,354,000
Financial assets:
Loans	2,943,000	2,998,000
Derivative assets	37,000	41,000
Long-term Debt	4,434,000	4,493,000
Financial liabilities:
Derivative liabilities	1,800,000	1,835,000
Long-term debt	4,270,000	4,324,000
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	3,175,000	3,215,000
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	140,000	139,000
Variable Interest Entity, Primary Beneficiary [Member] | Reported Value Measurement [Member]
Financial assets:
Restricted Cash and Cash Equivalents, Current	2,000	2,000
Loans	2,943,000	2,998,000
Financial liabilities:
Long-term debt	4,434,000	4,493,000
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	3,175,000	3,215,000
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	140,000	139,000
Variable Interest Entity, Primary Beneficiary [Member] | Reported Value Measurement [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities	1,800,000	1,835,000
Variable Interest Entity, Primary Beneficiary [Member] | Reported Value Measurement [Member] | Currency Swaps [Member]
Financial assets:
Derivative assets	37,000	41,000
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	4,299,000	4,349,000
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	140,000	139,000
Variable Interest Entity, Primary Beneficiary [Member] | Level 2 [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities	1,800,000	1,835,000
Variable Interest Entity, Primary Beneficiary [Member] | Level 2 [Member] | Currency Swaps [Member]
Financial assets:
Derivative assets	37,000	41,000
Variable Interest Entity, Primary Beneficiary [Member] | Level 3 [Member]
Financial assets:
Restricted Cash and Cash Equivalents, Current	0	0
Loans	2,943,000	2,998,000
Financial liabilities:
Long-term debt	173,000	155,000
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	3,175,000	3,215,000
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	2,943,000	2,998,000
Financial liabilities:
Long-term debt	4,472,000	4,504,000
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	3,175,000	3,215,000
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	140,000	139,000
Variable Interest Entity, Primary Beneficiary [Member] | Fair Value, Recurring [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities	1,800,000	1,835,000
Variable Interest Entity, Primary Beneficiary [Member] | Fair Value, Recurring [Member] | Currency Swaps [Member]
Financial assets:
Derivative assets	$ 37,000	$ 41,000