Fair Value Measurements - Summary of Carrying Amount and Fair Value of Ambac's Financial Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Financial assets:
Debt Securities, Available-for-sale	$ 3,314,000
Short term investments	653,000	$ 430,000
Other Investments	478,000	391,000
Derivative assets	75,000	60,000
Other assets	3,000	5,000
Financial liabilities:
Derivative liabilities	90,000	77,000
Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	3,121,000	2,737,000
Loans, at fair value	3,108,000	4,288,000
Derivative assets	52,000	66,000
Financial liabilities:
Derivative liabilities	1,657,000	1,712,000
Long-term Debt	4,351,000	5,269,000
Short-term Investments [Member]
Financial assets:
Debt Securities, Available-for-sale	653,000	430,000
Available-for-sale Securities	737,000
Corporate Debt Securities [Member] | Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	2,957,000	2,737,000
Municipal Bonds [Member] | Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	164,000	0
Carrying Amount [Member]
Financial assets:
Cash	79,000	82,000
Loans	10,000	10,000
Other assets	3,000	5,000
Total financial assets	10,242,000	11,186,000
Financial liabilities:
Liabilities for net financial guarantees written	(863,000)	(718,000)
Long-term Debt	3,262,000	3,305,000
Total financial liabilities	8,699,000	9,644,000
Carrying Amount [Member] | Variable Interest Entity [Member]
Financial assets:
Restricted cash	2,000	1,000
Loans, at fair value	3,108,000	4,288,000
Financial liabilities:
Long-term Debt	4,554,000	5,269,000
Carrying Amount [Member] | Other Debt Obligations [Member]
Financial assets:
Other Investments	478,000	391,000
Carrying Amount [Member] | Short-term Investments [Member]
Financial assets:
Short term investments	653,000	430,000
Carrying Amount [Member] | Other Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities		1,000
Carrying Amount [Member] | Corporate Debt Securities [Member] | Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	2,957,000	2,737,000
Carrying Amount [Member] | Municipal Bonds [Member] | Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	164,000
Carrying Amount [Member] | Interest Rate Swaps [Member]
Financial liabilities:
Derivative liabilities	89,000	72,000
Carrying Amount [Member] | Interest Rate Swaps [Member] | Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities	1,657,000	1,712,000
Carrying Amount [Member] | Futures Contracts [Member]
Financial liabilities:
Derivative liabilities		3,000
Carrying Amount [Member] | Currency Swaps [Member] | Variable Interest Entity [Member]
Financial assets:
Derivative assets	52,000	66,000
Total Fair Value [Member]
Financial assets:
Cash	79,000	82,000
Other Assets [Member] | Carrying Amount [Member] | Interest Rate Swaps [Member]
Financial assets:
Derivative assets	75,000	59,000
Fair Value Measured at Net Asset Value Per Share [Member]
Financial assets:
Other Investments	296,000	280,000
Level 1 [Member]
Financial assets:
Cash	70,000	53,000
Total financial assets	1,091,000	554,000
Financial liabilities:
Liabilities for net financial guarantees written	0	0
Long-term Debt	0	0
Total financial liabilities	0	3,000
Level 1 [Member] | Variable Interest Entity [Member]
Financial assets:
Restricted cash	2,000	1,000
Loans	0	0
Financial liabilities:
Long-term Debt	0	0
Level 1 [Member] | Other Debt Obligations [Member]
Financial assets:
Other Investments	136,000	71,000
Level 1 [Member] | Short-term Investments [Member]
Financial assets:
Short term investments	598,000	305,000
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
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] | Futures Contracts [Member]
Financial liabilities:
Derivative liabilities		3,000
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	9,000	30,000
Total financial assets	2,593,000	3,153,000
Financial liabilities:
Liabilities for net financial guarantees written	0	0
Long-term Debt	2,829,000	2,909,000
Total financial liabilities	8,983,000	9,745,000
Level 2 [Member] | Variable Interest Entity [Member]
Financial assets:
Restricted cash	0	0
Loans	0	0
Financial liabilities:
Long-term Debt	4,408,000	5,052,000
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	55,000	125,000
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	164,000
Level 2 [Member] | Interest Rate Swaps [Member]
Financial liabilities:
Derivative liabilities	89,000	72,000
Level 2 [Member] | Interest Rate Swaps [Member] | Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities	1,657,000	1,712,000
Level 2 [Member] | Futures Contracts [Member]
Financial liabilities:
Derivative liabilities		0
Level 2 [Member] | Currency Swaps [Member] | Variable Interest Entity [Member]
Financial assets:
Derivative assets	52,000	66,000
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	8,000	12,000
Level 3 [Member]
Financial assets:
Cash	0	0
Total financial assets	6,281,000	7,177,000
Financial liabilities:
Liabilities for net financial guarantees written	284,000	559,000
Long-term Debt	445,000	351,000
Total financial liabilities	889,000	1,128,000
Level 3 [Member] | Variable Interest Entity [Member]
Financial assets:
Restricted cash	0	0
Loans	3,108,000	4,288,000
Financial liabilities:
Long-term Debt	159,000	217,000
Level 3 [Member] | Other Debt Obligations [Member]
Financial assets:
Other Investments	61,000	16,000
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	3,000	5,000
Level 3 [Member] | Other Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities		1,000
Level 3 [Member] | Corporate Debt Securities [Member] | Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	2,957,000	2,737,000
Level 3 [Member] | Municipal Bonds [Member] | Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	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] | Futures Contracts [Member]
Financial liabilities:
Derivative liabilities
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	13,000	12,000
Level 3 [Member] | Other Assets [Member] | Interest Rate Swaps [Member]
Financial assets:
Derivative assets	67,000	47,000
Commercial Mortgage Backed Securities [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	50,000
Municipal Bonds [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	215,000	880,000
Corporate Debt Securities [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	1,430,000	1,278,000
Debt Security, Government, Non-US [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	44,000	31,000
US Government Debt Securities [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	156,000	94,000
Residential Mortgage-Backed Securities [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	248,000	259,000
Collateralized Debt Obligations [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	146,000	131,000
Asset-backed Securities [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	287,000	442,000
Fair Value, Recurring [Member]
Financial assets:
Total financial assets	10,260,000	11,164,000
Financial liabilities:
Liabilities for net financial guarantees written	284,000	559,000
Long-term Debt	3,274,000	3,260,000
Total financial liabilities	9,872,000	10,876,000
Fair Value, Recurring [Member] | Variable Interest Entity [Member]
Financial assets:
Restricted cash	2,000	1,000
Loans	3,108,000	4,288,000
Financial liabilities:
Long-term Debt	4,567,000	5,269,000
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	493,000	367,000
Fair Value, Recurring [Member] | Short-term Investments [Member]
Financial assets:
Short term investments	653,000	430,000
Fair Value, Recurring [Member] | Equity Method Investments [Member]
Financial assets:
Other assets	3,000	5,000
Fair Value, Recurring [Member] | Other Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities		1,000
Fair Value, Recurring [Member] | Asset-backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	287,000	442,000
Fair Value, Recurring [Member] | Collateralized Debt Obligations [Member]
Financial assets:
Debt Securities, Available-for-sale	146,000	131,000
Fair Value, Recurring [Member] | Residential Mortgage-Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	248,000	259,000
Fair Value, Recurring [Member] | US Government Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	156,000	94,000
Fair Value, Recurring [Member] | Debt Security, Government, Non-US [Member]
Financial assets:
Debt Securities, Available-for-sale	44,000	31,000
Fair Value, Recurring [Member] | Corporate Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	1,430,000	1,278,000
Fair Value, Recurring [Member] | Corporate Debt Securities [Member] | Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	2,957,000	2,737,000
Fair Value, Recurring [Member] | Municipal Bonds [Member]
Financial assets:
Debt Securities, Available-for-sale	215,000	880,000
Fair Value, Recurring [Member] | Municipal Bonds [Member] | Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	164,000
Fair Value, Recurring [Member] | Interest Rate Swaps [Member]
Financial liabilities:
Derivative liabilities	89,000	72,000
Fair Value, Recurring [Member] | Interest Rate Swaps [Member] | Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities	1,657,000	1,712,000
Fair Value, Recurring [Member] | Futures Contracts [Member]
Financial liabilities:
Derivative liabilities		3,000
Fair Value, Recurring [Member] | Currency Swaps [Member] | Variable Interest Entity [Member]
Financial assets:
Derivative assets	52,000	66,000
Fair Value, Recurring [Member] | Loans Receivable [Member]
Financial assets:
Loans	13,000	12,000
Fair Value, Recurring [Member] | Other Assets [Member] | Interest Rate Swaps [Member]
Financial assets:
Derivative assets	75,000	59,000
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	156,000	94,000
Fair Value, Recurring [Member] | Level 1 [Member] | Debt Security, Government, Non-US [Member]
Financial assets:
Debt Securities, Available-for-sale	44,000	30,000
Fair Value, Recurring [Member] | Level 1 [Member] | Corporate Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	0	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	215,000	370,000
Fair Value, Recurring [Member] | Level 2 [Member] | Collateralized Debt Obligations [Member]
Financial assets:
Debt Securities, Available-for-sale	146,000	131,000
Fair Value, Recurring [Member] | Level 2 [Member] | Residential Mortgage-Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	248,000	259,000
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	1,000
Fair Value, Recurring [Member] | Level 2 [Member] | Corporate Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	1,430,000	1,278,000
Fair Value, Recurring [Member] | Level 2 [Member] | Municipal Bonds [Member]
Financial assets:
Debt Securities, Available-for-sale	215,000	880,000
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	72,000	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] | Short-term Investments [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	85,000
Collateral Pledged [Member] | Fair Value, Recurring [Member] | Short-term Investments [Member]
Financial assets:
Debt Securities, Available-for-sale	85,000
Collateral Pledged [Member] | Fair Value, Recurring [Member] | Level 1 [Member] | Short-term Investments [Member]
Financial assets:
Debt Securities, Available-for-sale	85,000
Collateral Pledged [Member] | Fair Value, Recurring [Member] | Level 2 [Member] | Short-term Investments [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