Fair Value Measurements - Summary of Carrying Amount and Fair Value of Ambac's Financial Assets and Liabilities (Detail) - Successor [Member] - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Financial assets:
Fixed income securities	$ 5,309,161,000	$ 6,049,908,000
Short term investments	557,270,000	430,788,000
Other Investments	431,630,000	450,307,000
Other assets	5,979,000	7,382,000
Financial liabilities:
Borrowings under Guaranteed Investment Agreements	0	82,358,000
Long term debt, including accrued interest	991,696,000	1,114,405,000
Variable Interest Entity [Member]
Financial assets:
Loans at Carrying Amount	11,529,384,000	10,658,963,000
Financial liabilities:
Long term debt, including accrued interest	12,160,544,000	11,155,936,000
Carrying Amount [Member]
Financial assets:
Cash	623,703,000	91,025,000
Loans	10,358,000	4,160,000
Other assets	5,979,000	7,382,000
Total financial assets	20,853,695,000	20,047,333,000
Financial liabilities:
Borrowings under Guaranteed Investment Agreements		82,358,000
Long term debt, including accrued interest		1,536,352,000
Liabilities for net financial guarantees written	3,435,438,000	3,009,943,000
Long-term debt	1,428,680,000
Total financial liabilities	19,312,708,000	18,182,476,000
Carrying Amount [Member] | Variable Interest Entity [Member]
Financial assets:
Restricted cash	978,000	4,873,000
Loans at Carrying Amount	11,529,384,000	10,658,963,000
Financial liabilities:
Long-term debt	12,160,544,000	11,155,936,000
Carrying Amount [Member] | Futures Contracts [Member]
Financial assets:
Derivative assets		536,000
Carrying Amount [Member] | Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities		15,349,000
Total Fair Value [Member]
Financial assets:
Cash	623,703,000	91,025,000
Loans	10,284,000	4,066,000
Other assets	5,979,000	7,382,000
Total financial assets	20,835,968,000	20,032,169,000
Financial liabilities:
Obligations under investment and repurchase agreements		82,333,000
Long term debt, including accrued interest		1,494,340,000
Liabilities for net financial guarantees written	4,842,402,000	4,490,070,000
Long-term debt	1,369,499,000
Total financial liabilities	20,660,491,000	19,620,566,000
Total Fair Value [Member] | Variable Interest Entity [Member]
Financial assets:
Restricted cash	978,000	4,873,000
Loans	11,529,384,000	10,658,963,000
Financial liabilities:
Long-term debt	12,160,544,000	11,155,936,000
Total Fair Value [Member] | Futures Contracts [Member]
Financial assets:
Derivative assets		536,000
Total Fair Value [Member] | Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities		15,349,000
Other Assets [Member] | Carrying Amount [Member] | Interest Rate Swaps [Member]
Financial assets:
Derivative assets	73,199,000	77,206,000
Financial liabilities:
Derivative liabilities - interest rate swaps - asset position		(61,839,000)
Other Assets [Member] | Total Fair Value [Member] | Interest Rate Swaps [Member]
Financial assets:
Derivative assets	73,199,000	77,206,000
Financial liabilities:
Derivative liabilities - interest rate swaps - asset position		(61,839,000)
Interest Rate Swaps - Liability Position [Member] | Carrying Amount [Member] | Interest Rate Swaps [Member]
Financial assets:
Derivative assets - interest rate swaps - liability position		0
Financial liabilities:
Derivative liabilities		365,776,000
Interest Rate Swaps - Liability Position [Member] | Carrying Amount [Member] | Interest Rate Swaps [Member] | Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities		2,078,601,000
Interest Rate Swaps - Liability Position [Member] | Carrying Amount [Member] | Currency Swaps [Member] | Variable Interest Entity [Member]
Financial assets:
Derivative assets	54,877,000	80,407,000
Financial liabilities:
Derivative liabilities		0
Interest Rate Swaps - Liability Position [Member] | Total Fair Value [Member] | Interest Rate Swaps [Member]
Financial assets:
Derivative assets - interest rate swaps - liability position		0
Financial liabilities:
Derivative liabilities		365,776,000
Interest Rate Swaps - Liability Position [Member] | Total Fair Value [Member] | Interest Rate Swaps [Member] | Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities		2,078,601,000
Interest Rate Swaps - Liability Position [Member] | Total Fair Value [Member] | Currency Swaps [Member] | Variable Interest Entity [Member]
Financial assets:
Derivative assets	54,877,000	80,407,000
Financial liabilities:
Derivative liabilities		0
Level 1 [Member]
Financial assets:
Cash	615,073,000	46,587,000
Loans	0	0
Other assets	0	0
Total financial assets	1,173,321,000	650,457,000
Financial liabilities:
Obligations under investment and repurchase agreements		0
Long term debt, including accrued interest		0
Liabilities for net financial guarantees written	0	0
Long-term debt	0
Total financial liabilities	1,348,000	0
Level 1 [Member] | Variable Interest Entity [Member]
Financial assets:
Restricted cash	978,000	4,873,000
Loans	0	0
Financial liabilities:
Long-term debt	0	0
Level 1 [Member] | Futures Contracts [Member]
Financial assets:
Derivative assets		536,000
Level 1 [Member] | Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities		0
Level 1 [Member] | Other Assets [Member] | Interest Rate Swaps [Member]
Financial assets:
Derivative assets	0	0
Financial liabilities:
Derivative liabilities - interest rate swaps - asset position		0
Level 1 [Member] | Interest Rate Swaps - Liability Position [Member] | Interest Rate Swaps [Member]
Financial assets:
Derivative assets - interest rate swaps - liability position		0
Financial liabilities:
Derivative liabilities		0
Level 1 [Member] | Interest Rate Swaps - Liability Position [Member] | Interest Rate Swaps [Member] | Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities		0
Level 1 [Member] | Interest Rate Swaps - Liability Position [Member] | Currency Swaps [Member] | Variable Interest Entity [Member]
Financial assets:
Derivative assets	0	0
Financial liabilities:
Derivative liabilities		0
Level 2 [Member]
Financial assets:
Cash	8,630,000	44,438,000
Loans	0	0
Other assets	0	0
Total financial assets	4,005,195,000	4,914,330,000
Financial liabilities:
Obligations under investment and repurchase agreements		0
Long term debt, including accrued interest		1,147,728,000
Liabilities for net financial guarantees written	0	0
Long-term debt	1,046,511,000
Total financial liabilities	12,735,499,000	11,958,793,000
Level 2 [Member] | Variable Interest Entity [Member]
Financial assets:
Restricted cash	0	0
Loans	0	0
Financial liabilities:
Long-term debt	9,402,856,000	8,573,716,000
Level 2 [Member] | Futures Contracts [Member]
Financial assets:
Derivative assets		0
Level 2 [Member] | Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities		0
Level 2 [Member] | Other Assets [Member] | Interest Rate Swaps [Member]
Financial assets:
Derivative assets	11,825,000	16,950,000
Financial liabilities:
Derivative liabilities - interest rate swaps - asset position		(61,839,000)
Level 2 [Member] | Interest Rate Swaps - Liability Position [Member] | Interest Rate Swaps [Member]
Financial assets:
Derivative assets - interest rate swaps - liability position		0
Financial liabilities:
Derivative liabilities		220,587,000
Level 2 [Member] | Interest Rate Swaps - Liability Position [Member] | Interest Rate Swaps [Member] | Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities		2,078,601,000
Level 2 [Member] | Interest Rate Swaps - Liability Position [Member] | Currency Swaps [Member] | Variable Interest Entity [Member]
Financial assets:
Derivative assets	54,877,000	80,407,000
Financial liabilities:
Derivative liabilities		0
Level 3 [Member]
Financial assets:
Cash	0	0
Loans	10,284,000	4,066,000
Other assets	5,979,000	7,382,000
Total financial assets	15,347,011,000	14,130,870,000
Financial liabilities:
Obligations under investment and repurchase agreements		82,333,000
Long term debt, including accrued interest		346,612,000
Liabilities for net financial guarantees written	4,842,402,000	4,490,070,000
Long-term debt	322,988,000
Total financial liabilities	7,923,644,000	7,661,773,000
Level 3 [Member] | Variable Interest Entity [Member]
Financial assets:
Restricted cash	0	0
Loans	11,529,384,000	10,658,963,000
Financial liabilities:
Long-term debt	2,757,688,000	2,582,220,000
Level 3 [Member] | Futures Contracts [Member]
Financial assets:
Derivative assets		0
Level 3 [Member] | Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities		15,349,000
Level 3 [Member] | Other Assets [Member] | Interest Rate Swaps [Member]
Financial assets:
Derivative assets	61,374,000	60,256,000
Financial liabilities:
Derivative liabilities - interest rate swaps - asset position		0
Level 3 [Member] | Interest Rate Swaps - Liability Position [Member] | Interest Rate Swaps [Member]
Financial assets:
Derivative assets - interest rate swaps - liability position		0
Financial liabilities:
Derivative liabilities		145,189,000
Level 3 [Member] | Interest Rate Swaps - Liability Position [Member] | Interest Rate Swaps [Member] | Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities		0
Level 3 [Member] | Interest Rate Swaps - Liability Position [Member] | Currency Swaps [Member] | Variable Interest Entity [Member]
Financial assets:
Derivative assets	0	0
Financial liabilities:
Derivative liabilities		0
Ambac insured Re-remic [Member] | Level 3 [Member]
Financial assets:
Fixed income securities		0
Municipal Bonds [Member] | Carrying Amount [Member]
Financial assets:
Fixed income securities	779,834,000	374,368,000
Municipal Bonds [Member] | Total Fair Value [Member]
Financial assets:
Fixed income securities	779,834,000	374,368,000
Municipal Bonds [Member] | Level 1 [Member]
Financial assets:
Fixed income securities	0	0
Municipal Bonds [Member] | Level 2 [Member]
Financial assets:
Fixed income securities	779,834,000	374,368,000
Municipal Bonds [Member] | Level 3 [Member]
Financial assets:
Fixed income securities	0	0
Corporate Obligations [Member] | Carrying Amount [Member]
Financial assets:
Fixed income securities	860,075,000	1,802,165,000
Corporate Obligations [Member] | Carrying Amount [Member] | Variable Interest Entity [Member]
Financial assets:
Fixed income securities	2,914,145,000	2,622,566,000
Corporate Obligations [Member] | Total Fair Value [Member]
Financial assets:
Fixed income securities	860,075,000	1,802,165,000
Corporate Obligations [Member] | Total Fair Value [Member] | Variable Interest Entity [Member]
Financial assets:
Fixed income securities	2,914,145,000	2,622,566,000
Corporate Obligations [Member] | Level 1 [Member]
Financial assets:
Fixed income securities	450,000	0
Corporate Obligations [Member] | Level 1 [Member] | Variable Interest Entity [Member]
Financial assets:
Fixed income securities	0	0
Corporate Obligations [Member] | Level 2 [Member]
Financial assets:
Fixed income securities	859,625,000	1,802,165,000
Corporate Obligations [Member] | Level 2 [Member] | Variable Interest Entity [Member]
Financial assets:
Fixed income securities	0	0
Corporate Obligations [Member] | Level 3 [Member]
Financial assets:
Fixed income securities	0	0
Corporate Obligations [Member] | Level 3 [Member] | Variable Interest Entity [Member]
Financial assets:
Fixed income securities	2,914,145,000	2,622,566,000
Foreign Obligations [Member] | Carrying Amount [Member]
Financial assets:
Fixed income securities	26,543,000	43,135,000
Foreign Obligations [Member] | Total Fair Value [Member]
Financial assets:
Fixed income securities	26,543,000	43,135,000
Foreign Obligations [Member] | Level 1 [Member]
Financial assets:
Fixed income securities	25,615,000	42,212,000
Foreign Obligations [Member] | Level 2 [Member]
Financial assets:
Fixed income securities	928,000	923,000
Foreign Obligations [Member] | Level 3 [Member]
Financial assets:
Fixed income securities	0	0
US Government Debt Securities [Member] | Carrying Amount [Member]
Financial assets:
Fixed income securities	85,408,000	36,186,000
Fixed income securities, pledged as collateral	99,719,000	64,905,000
US Government Debt Securities [Member] | Total Fair Value [Member]
Financial assets:
Fixed income securities	85,408,000	36,186,000
Fixed income securities, pledged as collateral	99,719,000	64,905,000
US Government Debt Securities [Member] | Level 1 [Member]
Financial assets:
Fixed income securities	85,408,000	36,186,000
Fixed income securities, pledged as collateral	99,719,000	64,905,000
US Government Debt Securities [Member] | Level 2 [Member]
Financial assets:
Fixed income securities	0	0
Fixed income securities, pledged as collateral	0	0
US Government Debt Securities [Member] | Level 3 [Member]
Financial assets:
Fixed income securities	0	0
Fixed income securities, pledged as collateral	0	0
U.S. Agency Obligations [Member] | Carrying Amount [Member]
Financial assets:
Fixed income securities		4,060,000
U.S. Agency Obligations [Member] | Total Fair Value [Member]
Financial assets:
Fixed income securities		4,060,000
U.S. Agency Obligations [Member] | Level 1 [Member]
Financial assets:
Fixed income securities		0
U.S. Agency Obligations [Member] | Level 2 [Member]
Financial assets:
Fixed income securities		4,060,000
U.S. Agency Obligations [Member] | Level 3 [Member]
Financial assets:
Fixed income securities		0
Residential Mortgage-Backed Securities [Member] | Carrying Amount [Member]
Financial assets:
Fixed income securities	2,251,333,000	2,351,595,000
Residential Mortgage-Backed Securities [Member] | Total Fair Value [Member]
Financial assets:
Fixed income securities	2,251,333,000	2,351,595,000
Residential Mortgage-Backed Securities [Member] | Level 1 [Member]
Financial assets:
Fixed income securities	0	0
Residential Mortgage-Backed Securities [Member] | Level 2 [Member]
Financial assets:
Fixed income securities	1,515,316,000	1,654,882,000
Residential Mortgage-Backed Securities [Member] | Level 3 [Member]
Financial assets:
Fixed income securities	736,017,000	696,713,000
RMBS Priced Using Discounted Cash Flow Approach [Member] | Level 3 [Member]
Financial assets:
Fixed income securities	26,067,000
Collateralized Debt Obligations [Member] | Carrying Amount [Member]
Financial assets:
Fixed income securities	51,037,000	113,923,000
Collateralized Debt Obligations [Member] | Total Fair Value [Member]
Financial assets:
Fixed income securities	51,037,000	113,923,000
Collateralized Debt Obligations [Member] | Level 1 [Member]
Financial assets:
Fixed income securities	0	0
Collateralized Debt Obligations [Member] | Level 2 [Member]
Financial assets:
Fixed income securities	51,037,000	113,923,000
Collateralized Debt Obligations [Member] | Level 3 [Member]
Financial assets:
Fixed income securities	0	0
Asset-backed Securities [Member] | Carrying Amount [Member]
Financial assets:
Fixed income securities	597,942,000	828,783,000
Asset-backed Securities [Member] | Total Fair Value [Member]
Financial assets:
Fixed income securities	597,942,000	828,783,000
Asset-backed Securities [Member] | Level 1 [Member]
Financial assets:
Fixed income securities	0	0
Asset-backed Securities [Member] | Level 2 [Member]
Financial assets:
Fixed income securities	525,402,000	762,793,000
Asset-backed Securities [Member] | Level 3 [Member]
Financial assets:
Fixed income securities	72,540,000	65,990,000
Short-Term [Member] | Carrying Amount [Member]
Financial assets:
Short term investments	557,270,000	430,788,000
Short-Term [Member] | Total Fair Value [Member]
Financial assets:
Short term investments	557,270,000	430,788,000
Short-Term [Member] | Level 1 [Member]
Financial assets:
Short term investments	389,299,000	371,367,000
Short-Term [Member] | Level 2 [Member]
Financial assets:
Short term investments	167,971,000	59,421,000
Short-Term [Member] | Level 3 [Member]
Financial assets:
Short term investments	0	0
Other [Member] | Carrying Amount [Member]
Financial assets:
Other Investments	431,630,000	450,307,000
Other [Member] | Total Fair Value [Member]
Financial assets:
Other Investments	413,977,000	435,237,000
Other [Member] | Level 1 [Member]
Financial assets:
Other Investments	56,498,000	83,791,000
Other [Member] | Level 2 [Member]
Financial assets:
Other Investments	29,750,000	0
Other [Member] | Level 3 [Member]
Financial assets:
Other Investments	17,288,000	14,934,000
Investments Measured at NAV as a Practical Expedient [Member]
Financial assets:
Other Investments	310,441,000	$ 336,513,000
Futures Contracts [Member] | Carrying Amount [Member]
Financial liabilities:
Derivative liabilities	1,348,000
Futures Contracts [Member] | Total Fair Value [Member]
Financial liabilities:
Derivative liabilities	1,348,000
Futures Contracts [Member] | Level 1 [Member]
Financial liabilities:
Derivative liabilities	1,348,000
Futures Contracts [Member] | Level 2 [Member]
Financial liabilities:
Derivative liabilities	0
Futures Contracts [Member] | Level 3 [Member]
Financial liabilities:
Derivative liabilities	0
Interest Rate Swaps [Member] | Carrying Amount [Member]
Financial liabilities:
Derivative liabilities - interest rate swaps - asset position	627,000
Derivative liabilities	81,495,000
Interest Rate Swaps [Member] | Carrying Amount [Member] | Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities	2,205,264,000
Interest Rate Swaps [Member] | Total Fair Value [Member]
Financial liabilities:
Derivative liabilities - interest rate swaps - asset position	627,000
Derivative liabilities	81,495,000
Interest Rate Swaps [Member] | Total Fair Value [Member] | Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities	2,205,264,000
Interest Rate Swaps [Member] | Level 1 [Member]
Financial liabilities:
Derivative liabilities - interest rate swaps - asset position	0
Derivative liabilities	0
Interest Rate Swaps [Member] | Level 1 [Member] | Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities	0
Interest Rate Swaps [Member] | Level 2 [Member]
Financial liabilities:
Derivative liabilities - interest rate swaps - asset position	627,000
Derivative liabilities	81,495,000
Interest Rate Swaps [Member] | Level 2 [Member] | Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities	2,205,264,000
Interest Rate Swaps [Member] | Level 3 [Member]
Financial liabilities:
Derivative liabilities - interest rate swaps - asset position	0
Derivative liabilities	0
Interest Rate Swaps [Member] | Level 3 [Member] | Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities	0
Credit Derivatives [Member] | Carrying Amount [Member] | Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities	566,000
Credit Derivatives [Member] | Total Fair Value [Member] | Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities	566,000
Credit Derivatives [Member] | Level 1 [Member] | Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities	0
Credit Derivatives [Member] | Level 2 [Member] | Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities	0
Credit Derivatives [Member] | Level 3 [Member] | Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities	$ 566,000