Fair Value Measurements - Summary of Carrying Amount and Fair Value of Ambac's Financial Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Carrying Amount [Member]
Financial assets:
Other Investments	$ 89,000	$ 136,000
Cash	33,000	79,000
Loans	3,000	10,000
Other assets	1,000	3,000
Total financial assets	10,071,000	10,242,000
Financial liabilities:
Liabilities for net financial guarantees written	(740,000)	(863,000)
Long-term Debt	3,255,000	3,262,000
Total financial liabilities	8,958,000	8,699,000
Carrying Amount [Member] | Variable Interest Entity [Member]
Financial assets:
Restricted cash	2,000	2,000
Loans, at fair value	2,998,000	3,108,000
Financial liabilities:
Long-term Debt	4,493,000	4,554,000
Carrying Amount [Member] | Other Debt Obligations [Member]
Financial assets:
Other Investments	595,000	478,000
Carrying Amount [Member] | Short-term Investments [Member]
Financial assets:
Short term investments	492,000	653,000
Carrying Amount [Member] | Other Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities		0
Carrying Amount [Member] | Corporate Debt Securities [Member] | Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	3,215,000	2,957,000
Carrying Amount [Member] | Municipal Bonds [Member] | Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	139,000	164,000
Carrying Amount [Member] | Interest Rate Swaps [Member]
Financial liabilities:
Derivative liabilities	114,000	89,000
Carrying Amount [Member] | Interest Rate Swaps [Member] | Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities	1,835,000	1,657,000
Carrying Amount [Member] | Currency Swaps [Member] | Variable Interest Entity [Member]
Financial assets:
Derivative assets	41,000	52,000
Total Fair Value [Member]
Financial assets:
Cash	33,000	79,000
Debt Securities, Available-for-sale	2,949,000	3,314,000
Other Investments	595,000	478,000
Derivative assets	93,000	75,000
Other assets	1,000	3,000
Financial liabilities:
Derivative liabilities	114,000	90,000
Credit valuation adjustment included in fair value of credit derivative liabilities	$ 32	$ 19
Estimated Future Premium Payments Weighted Average Discounted Rate	2.20%	2.40%
Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	$ 3,354,000	$ 3,121,000
Loans, at fair value	2,998,000	3,108,000
Derivative assets	41,000	52,000
Financial liabilities:
Derivative liabilities	1,835,000	1,657,000
Long-term Debt	$ 4,324,000	$ 4,351,000
Variable Interest Entity [Member]
Financial liabilities:
Estimated Future Premium Payments Weighted Average Discounted Rate	2.40%	2.70%
Short-term Investments [Member]
Financial assets:
Debt Securities, Available-for-sale	$ 492,000
Corporate Debt Securities [Member] | Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	3,215,000	$ 2,957,000
Municipal Bonds [Member] | Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	139,000	164,000
Other Assets [Member] | Carrying Amount [Member] | Interest Rate Swaps [Member]
Financial assets:
Derivative assets	93,000	75,000
Fair Value Measured at Net Asset Value Per Share [Member]
Financial assets:
Other Investments	453,000	296,000
Level 1 [Member]
Financial assets:
Cash	32,000	70,000
Total financial assets	888,000	1,091,000
Financial liabilities:
Liabilities for net financial guarantees written	0	0
Long-term Debt	0	0
Total financial liabilities	0	0
Level 1 [Member] | Variable Interest Entity [Member]
Financial assets:
Restricted cash	2,000	2,000
Loans	0	0
Financial liabilities:
Long-term Debt	0	0
Level 1 [Member] | Other Debt Obligations [Member]
Financial assets:
Other Investments	91,000	136,000
Level 1 [Member] | Short-term Investments [Member]
Financial assets:
Short term investments	415,000	598,000
Level 1 [Member] | Equity Method Investments [Member]
Financial assets:
Other assets	0	0
Level 1 [Member] | Other Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities		0
Level 1 [Member] | Corporate Debt Securities [Member] | Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Level 1 [Member] | Municipal Bonds [Member] | Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Level 1 [Member] | Interest Rate Swaps [Member]
Financial liabilities:
Derivative liabilities	0	0
Level 1 [Member] | Interest Rate Swaps [Member] | Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities	0	0
Level 1 [Member] | Currency Swaps [Member] | Variable Interest Entity [Member]
Financial assets:
Derivative assets	0	0
Level 1 [Member] | Loans Receivable [Member]
Financial assets:
Loans	0	0
Level 1 [Member] | Other Assets [Member] | Interest Rate Swaps [Member]
Financial assets:
Derivative assets	0	0
Level 2 [Member]
Financial assets:
Cash	2,000	9,000
Total financial assets	2,299,000	2,593,000
Financial liabilities:
Liabilities for net financial guarantees written	0	0
Long-term Debt	2,670,000	2,829,000
Total financial liabilities	8,968,000	8,983,000
Level 2 [Member] | Variable Interest Entity [Member]
Financial assets:
Restricted cash	0	0
Loans	0	0
Financial liabilities:
Long-term Debt	4,349,000	4,408,000
Level 2 [Member] | Other Debt Obligations [Member]
Financial assets:
Other Investments	0	0
Level 2 [Member] | Short-term Investments [Member]
Financial assets:
Short term investments	76,000	55,000
Level 2 [Member] | Equity Method Investments [Member]
Financial assets:
Other assets	0	0
Level 2 [Member] | Other Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities		0
Level 2 [Member] | Corporate Debt Securities [Member] | Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Level 2 [Member] | Municipal Bonds [Member] | Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	139,000	164,000
Level 2 [Member] | Interest Rate Swaps [Member]
Financial liabilities:
Derivative liabilities	114,000	89,000
Level 2 [Member] | Interest Rate Swaps [Member] | Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities	1,835,000	1,657,000
Level 2 [Member] | Currency Swaps [Member] | Variable Interest Entity [Member]
Financial assets:
Derivative assets	41,000	52,000
Level 2 [Member] | Loans Receivable [Member]
Financial assets:
Loans	0	0
Level 2 [Member] | Other Assets [Member] | Interest Rate Swaps [Member]
Financial assets:
Derivative assets	9,000	8,000
Level 3 [Member]
Financial assets:
Cash	0	0
Total financial assets	6,433,000	6,281,000
Financial liabilities:
Liabilities for net financial guarantees written	539,000	284,000
Long-term Debt	401,000	445,000
Total financial liabilities	1,095,000	889,000
Level 3 [Member] | Variable Interest Entity [Member]
Financial assets:
Restricted cash	0	0
Loans	2,998,000	3,108,000
Financial liabilities:
Long-term Debt	155,000	159,000
Level 3 [Member] | Other Debt Obligations [Member]
Financial assets:
Other Investments	53,000	61,000
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	1,000	3,000
Level 3 [Member] | Other Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities		0
Level 3 [Member] | Corporate Debt Securities [Member] | Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	3,215,000	2,957,000
Level 3 [Member] | Municipal Bonds [Member] | Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Level 3 [Member] | Interest Rate Swaps [Member]
Financial liabilities:
Derivative liabilities	0	0
Level 3 [Member] | Interest Rate Swaps [Member] | Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities	0	0
Level 3 [Member] | Currency Swaps [Member] | Variable Interest Entity [Member]
Financial assets:
Derivative assets	0	0
Level 3 [Member] | Loans Receivable [Member]
Financial assets:
Loans	3,000	13,000
Level 3 [Member] | Other Assets [Member] | Interest Rate Swaps [Member]
Financial assets:
Derivative assets	85,000	67,000
Municipal Bonds [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	358,000	215,000
Corporate Debt Securities [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	1,077,000	1,430,000
Debt Security, Government, Non-US [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	98,000	44,000
US Government Debt Securities [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	106,000	156,000
Residential Mortgage-Backed Securities [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	302,000	248,000
Residential Mortgage-Backed Securities [Member] | Commercial Mortgage Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale		50,000
Collateralized Debt Obligations [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	74,000	146,000
Asset-backed Securities [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	303,000	287,000
Fair Value, Recurring [Member]
Financial assets:
Total financial assets	10,073,000	10,260,000
Financial liabilities:
Liabilities for net financial guarantees written	539,000	284,000
Long-term Debt	3,071,000	3,274,000
Total financial liabilities	10,063,000	9,872,000
Fair Value, Recurring [Member] | Variable Interest Entity [Member]
Financial assets:
Restricted cash	2,000	2,000
Loans	2,998,000	3,108,000
Financial liabilities:
Long-term Debt	4,504,000	4,567,000
Fair Value, Recurring [Member] | Commercial Mortgage Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale		50,000
Fair Value, Recurring [Member] | Other Debt Obligations [Member]
Financial assets:
Other Investments	597,000	493,000
Fair Value, Recurring [Member] | Short-term Investments [Member]
Financial assets:
Short term investments	492,000	653,000
Fair Value, Recurring [Member] | Equity Method Investments [Member]
Financial assets:
Other assets	1,000	3,000
Fair Value, Recurring [Member] | Other Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities		0
Fair Value, Recurring [Member] | Asset-backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	303,000	287,000
Fair Value, Recurring [Member] | Collateralized Debt Obligations [Member]
Financial assets:
Debt Securities, Available-for-sale	74,000	146,000
Fair Value, Recurring [Member] | Residential Mortgage-Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	302,000	248,000
Fair Value, Recurring [Member] | US Government Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	106,000	156,000
Fair Value, Recurring [Member] | Debt Security, Government, Non-US [Member]
Financial assets:
Debt Securities, Available-for-sale	98,000	44,000
Fair Value, Recurring [Member] | Corporate Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	1,077,000	1,430,000
Fair Value, Recurring [Member] | Corporate Debt Securities [Member] | Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	3,215,000	2,957,000
Fair Value, Recurring [Member] | Municipal Bonds [Member]
Financial assets:
Debt Securities, Available-for-sale	358,000	215,000
Fair Value, Recurring [Member] | Municipal Bonds [Member] | Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	139,000	164,000
Fair Value, Recurring [Member] | Interest Rate Swaps [Member]
Financial liabilities:
Derivative liabilities	114,000	89,000
Fair Value, Recurring [Member] | Interest Rate Swaps [Member] | Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities	1,835,000	1,657,000
Fair Value, Recurring [Member] | Currency Swaps [Member] | Variable Interest Entity [Member]
Financial assets:
Derivative assets	41,000	52,000
Fair Value, Recurring [Member] | Loans Receivable [Member]
Financial assets:
Loans	3,000	13,000
Fair Value, Recurring [Member] | Other Assets [Member] | Interest Rate Swaps [Member]
Financial assets:
Derivative assets	93,000	75,000
Fair Value, Recurring [Member] | Level 1 [Member] | Commercial Mortgage Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale		0
Fair Value, Recurring [Member] | Level 1 [Member] | Asset-backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] | Level 1 [Member] | Collateralized Debt Obligations [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] | Level 1 [Member] | Residential Mortgage-Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] | Level 1 [Member] | US Government Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	106,000	156,000
Fair Value, Recurring [Member] | Level 1 [Member] | Debt Security, Government, Non-US [Member]
Financial assets:
Debt Securities, Available-for-sale	98,000	44,000
Fair Value, Recurring [Member] | Level 1 [Member] | Corporate Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	4,000	0
Fair Value, Recurring [Member] | Level 1 [Member] | Municipal Bonds [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] | Level 2 [Member] | Commercial Mortgage Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale		50,000
Fair Value, Recurring [Member] | Level 2 [Member] | Asset-backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	225,000	215,000
Fair Value, Recurring [Member] | Level 2 [Member] | Collateralized Debt Obligations [Member]
Financial assets:
Debt Securities, Available-for-sale	74,000	146,000
Fair Value, Recurring [Member] | Level 2 [Member] | Residential Mortgage-Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	302,000	248,000
Fair Value, Recurring [Member] | Level 2 [Member] | US Government Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] | Level 2 [Member] | Debt Security, Government, Non-US [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] | Level 2 [Member] | Corporate Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	1,073,000	1,430,000
Fair Value, Recurring [Member] | Level 2 [Member] | Municipal Bonds [Member]
Financial assets:
Debt Securities, Available-for-sale	358,000	215,000
Fair Value, Recurring [Member] | Level 3 [Member] | Commercial Mortgage Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale		0
Fair Value, Recurring [Member] | Level 3 [Member] | Asset-backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	78,000	72,000
Fair Value, Recurring [Member] | Level 3 [Member] | Collateralized Debt Obligations [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] | Level 3 [Member] | Residential Mortgage-Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] | Level 3 [Member] | US Government Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] | Level 3 [Member] | Debt Security, Government, Non-US [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] | Level 3 [Member] | Corporate Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] | Level 3 [Member] | Municipal Bonds [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Collateral Pledged [Member]
Financial assets:
Debt Securities, Available-for-sale	140,000
Collateral Pledged [Member] | US Government Debt Securities [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	15,000
Collateral Pledged [Member] | Short-term Investments [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	125,000	85,000
Collateral Pledged [Member] | Fair Value, Recurring [Member] | Short-term Investments [Member]
Financial assets:
Debt Securities, Available-for-sale	125,000	85,000
Collateral Pledged [Member] | Fair Value, Recurring [Member] | US Government Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	15,000
Collateral Pledged [Member] | Fair Value, Recurring [Member] | Level 1 [Member] | Short-term Investments [Member]
Financial assets:
Debt Securities, Available-for-sale	125,000	85,000
Collateral Pledged [Member] | Fair Value, Recurring [Member] | Level 1 [Member] | US Government Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	15,000
Collateral Pledged [Member] | Fair Value, Recurring [Member] | Level 2 [Member] | Short-term Investments [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Collateral Pledged [Member] | Fair Value, Recurring [Member] | Level 2 [Member] | US Government Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	0
Collateral Pledged [Member] | Fair Value, Recurring [Member] | Level 3 [Member] | Short-term Investments [Member]
Financial assets:
Debt Securities, Available-for-sale	0	$ 0
Collateral Pledged [Member] | Fair Value, Recurring [Member] | Level 3 [Member] | US Government Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	$ 0