Fair Value Measurements - Summary of Carrying Amount and Fair Value of Ambac's Financial Assets and Liabilities (Detail) - Successor [Member] - USD ($)	Mar. 31, 2017	Dec. 31, 2016
Financial assets:
Fixed income securities	$ 5,855,962,000	$ 6,049,908,000
Short term investments	347,055,000	430,788,000
Other investments	478,688,000	450,307,000
Other assets	7,039,000	7,382,000
Borrowings under Guaranteed Investment Agreements	0	82,358,000
Variable Interest Entity [Member]
Financial assets:
Loans	11,015,305,000	10,658,963,000
Total Fair Value [Member]
Financial assets:
Cash	118,772,000	91,025,000
Loans	4,227,000	4,066,000
Other assets	7,039,000	7,382,000
Total financial assets	20,282,908,000	20,032,170,000
Financial liabilities:
Obligations under investment and repurchase agreements		82,333,000
Liabilities for net financial guarantees written	5,007,413,000	4,490,070,000
Long-term debt	1,375,493,000	1,494,340,000
Total financial liabilities	20,255,300,000	19,620,566,000
Total Fair Value [Member] | Variable Interest Entity [Member]
Financial assets:
Restricted cash	5,005,000	4,873,000
Loans	11,015,305,000	10,658,963,000
Financial liabilities:
Long-term debt	11,564,054,000	11,155,936,000
Total Fair Value [Member] | Future [Member]
Financial assets:
Derivative assets		536,000
Carrying Amount [Member]
Financial assets:
Cash	118,772,000	91,025,000
Loans	4,237,000	4,160,000
Other assets	7,039,000	7,382,000
Total financial assets	20,298,583,000	20,047,333,000
Borrowings under Guaranteed Investment Agreements		82,358,000
Financial liabilities:
Liabilities for net financial guarantees written	3,141,107,000	3,009,943,000
Long-term debt	1,412,450,000	1,536,352,000
Total financial liabilities	18,425,951,000	18,182,476,000
Carrying Amount [Member] | Variable Interest Entity [Member]
Financial assets:
Restricted cash	5,005,000	4,873,000
Loans	11,015,305,000	10,658,963,000
Financial liabilities:
Long-term debt	11,564,054,000	11,155,936,000
Carrying Amount [Member] | Future [Member]
Financial assets:
Derivative assets		536,000
Investments Measured at NAV as a Practical Expedient [Member]
Financial assets:
Other investments	334,689,000	336,513,000
Other Assets [Member] | Total Fair Value [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	74,397,000	77,206,000
Other Assets [Member] | Carrying Amount [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	74,397,000	77,206,000
Interest Rate Swaps - Liability Position [Member] | Total Fair Value [Member] | Currency Swaps [Member] | Variable Interest Entity [Member]
Financial assets:
Derivative assets	75,459,000	80,407,000
Interest Rate Swaps - Liability Position [Member] | Carrying Amount [Member] | Currency Swaps [Member] | Variable Interest Entity [Member]
Financial assets:
Derivative assets	75,459,000	80,407,000
Level 1 [Member]
Financial assets:
Cash	56,724,000	46,587,000
Loans	0	0
Other assets	0	0
Total financial assets	555,600,000	650,457,000
Financial liabilities:
Obligations under investment and repurchase agreements		0
Liabilities for net financial guarantees written	0	0
Long-term debt	0	0
Total financial liabilities	437,000	0
Level 1 [Member] | Variable Interest Entity [Member]
Financial assets:
Restricted cash	5,005,000	4,873,000
Loans	0	0
Financial liabilities:
Long-term debt	0	0
Level 1 [Member] | Future [Member]
Financial assets:
Derivative assets		536,000
Level 1 [Member] | Other Assets [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	0	0
Level 1 [Member] | Interest Rate Swaps - Liability Position [Member] | Currency Swaps [Member] | Variable Interest Entity [Member]
Financial assets:
Derivative assets	0	0
Level 2 [Member]
Financial assets:
Cash	62,048,000	44,438,000
Loans	0	0
Other assets	0	0
Total financial assets	4,906,227,000	4,914,330,000
Financial liabilities:
Obligations under investment and repurchase agreements		0
Liabilities for net financial guarantees written	0	0
Long-term debt	1,057,242,000	1,147,728,000
Total financial liabilities	12,051,027,000	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	8,847,412,000	8,573,716,000
Level 2 [Member] | Future [Member]
Financial assets:
Derivative assets		0
Level 2 [Member] | Other Assets [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	15,455,000	16,950,000
Level 2 [Member] | Interest Rate Swaps - Liability Position [Member] | Currency Swaps [Member] | Variable Interest Entity [Member]
Financial assets:
Derivative assets	75,459,000	80,407,000
Level 3 [Member]
Financial assets:
Cash	0	0
Loans	4,227,000	4,066,000
Other assets	7,039,000	7,382,000
Total financial assets	14,486,392,000	14,130,870,000
Financial liabilities:
Obligations under investment and repurchase agreements		82,333,000
Liabilities for net financial guarantees written	5,007,413,000	4,490,070,000
Long-term debt	318,251,000	346,612,000
Total financial liabilities	8,203,836,000	7,661,773,000
Level 3 [Member] | Variable Interest Entity [Member]
Financial assets:
Restricted cash	0	0
Loans	11,015,305,000	10,658,963,000
Financial liabilities:
Long-term debt	2,716,642,000	2,582,220,000
Level 3 [Member] | Future [Member]
Financial assets:
Derivative assets		0
Level 3 [Member] | Other Assets [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	58,942,000	60,256,000
Level 3 [Member] | Interest Rate Swaps - Liability Position [Member] | Currency Swaps [Member] | Variable Interest Entity [Member]
Financial assets:
Derivative assets	0	0
Municipal Bonds [Member] | Total Fair Value [Member]
Financial assets:
Fixed income securities	456,707,000	374,368,000
Municipal Bonds [Member] | Carrying Amount [Member]
Financial assets:
Fixed income securities	456,707,000	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	456,707,000	374,368,000
Municipal Bonds [Member] | Level 3 [Member]
Financial assets:
Fixed income securities	0	0
Corporate Obligations [Member] | Total Fair Value [Member]
Financial assets:
Fixed income securities	1,774,924,000	1,802,165,000
Corporate Obligations [Member] | Total Fair Value [Member] | Variable Interest Entity [Member]
Financial assets:
Fixed income securities	2,663,719,000	2,622,566,000
Corporate Obligations [Member] | Carrying Amount [Member]
Financial assets:
Fixed income securities	1,774,924,000	1,802,165,000
Corporate Obligations [Member] | Carrying Amount [Member] | Variable Interest Entity [Member]
Financial assets:
Fixed income securities	2,663,719,000	2,622,566,000
Corporate Obligations [Member] | Level 1 [Member]
Financial assets:
Fixed income securities	0	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	1,774,924,000	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	2,663,719,000	2,622,566,000
Foreign Obligations [Member] | Total Fair Value [Member]
Financial assets:
Fixed income securities	29,463,000	43,135,000
Foreign Obligations [Member] | Carrying Amount [Member]
Financial assets:
Fixed income securities	29,463,000	43,135,000
Foreign Obligations [Member] | Level 1 [Member]
Financial assets:
Fixed income securities	28,525,000	42,212,000
Foreign Obligations [Member] | Level 2 [Member]
Financial assets:
Fixed income securities	938,000	923,000
Foreign Obligations [Member] | Level 3 [Member]
Financial assets:
Fixed income securities	0	0
US Government Debt Securities [Member] | Total Fair Value [Member]
Financial assets:
Fixed income securities	42,009,000	36,186,000
Fixed income securities, pledged as collateral	64,908,000	64,905,000
US Government Debt Securities [Member] | Carrying Amount [Member]
Financial assets:
Fixed income securities	42,009,000	36,186,000
Fixed income securities, pledged as collateral	64,908,000	64,905,000
US Government Debt Securities [Member] | Level 1 [Member]
Financial assets:
Fixed income securities	42,009,000	36,186,000
Fixed income securities, pledged as collateral	64,908,000	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] | Total Fair Value [Member]
Financial assets:
Fixed income securities	4,018,000	4,060,000
U.S. Agency Obligations [Member] | Carrying Amount [Member]
Financial assets:
Fixed income securities	4,018,000	4,060,000
U.S. Agency Obligations [Member] | Level 1 [Member]
Financial assets:
Fixed income securities	0	0
U.S. Agency Obligations [Member] | Level 2 [Member]
Financial assets:
Fixed income securities	4,018,000	4,060,000
U.S. Agency Obligations [Member] | Level 3 [Member]
Financial assets:
Fixed income securities	0	0
Residential Mortgage-Backed Securities [Member] | Total Fair Value [Member]
Financial assets:
Fixed income securities	2,314,487,000	2,351,595,000
Residential Mortgage-Backed Securities [Member] | Carrying Amount [Member]
Financial assets:
Fixed income securities	2,314,487,000	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	1,658,670,000	1,654,882,000
Residential Mortgage-Backed Securities [Member] | Level 3 [Member]
Financial assets:
Fixed income securities	655,817,000	696,713,000
Collateralized Debt Obligations [Member] | Total Fair Value [Member]
Financial assets:
Fixed income securities	153,104,000	113,923,000
Collateralized Debt Obligations [Member] | Carrying Amount [Member]
Financial assets:
Fixed income securities	153,104,000	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	153,104,000	113,923,000
Collateralized Debt Obligations [Member] | Level 3 [Member]
Financial assets:
Fixed income securities	0	0
Asset-backed Securities [Member] | Total Fair Value [Member]
Financial assets:
Fixed income securities	669,287,000	828,783,000
Asset-backed Securities [Member] | Carrying Amount [Member]
Financial assets:
Fixed income securities	669,287,000	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	603,480,000	762,793,000
Asset-backed Securities [Member] | Level 3 [Member]
Financial assets:
Fixed income securities	65,807,000	65,990,000
Short-Term [Member] | Total Fair Value [Member]
Financial assets:
Short term investments	347,055,000	430,788,000
Short-Term [Member] | Carrying Amount [Member]
Financial assets:
Short term investments	347,055,000	430,788,000
Short-Term [Member] | Level 1 [Member]
Financial assets:
Short term investments	245,631,000	371,367,000
Short-Term [Member] | Level 2 [Member]
Financial assets:
Short term investments	101,424,000	59,421,000
Short-Term [Member] | Level 3 [Member]
Financial assets:
Short term investments	0	0
Other [Member] | Total Fair Value [Member]
Financial assets:
Other investments	463,023,000	435,238,000
Other [Member] | Carrying Amount [Member]
Financial assets:
Other investments	478,688,000	450,307,000
Other [Member] | Level 1 [Member]
Financial assets:
Other investments	112,798,000	83,791,000
Other [Member] | Level 2 [Member]
Financial assets:
Other investments	0	0
Other [Member] | Level 3 [Member]
Financial assets:
Other investments	15,536,000	14,934,000
Future [Member] | Total Fair Value [Member]
Financial liabilities:
Derivative liabilities	437,000
Future [Member] | Carrying Amount [Member]
Financial liabilities:
Derivative liabilities	437,000
Future [Member] | Level 1 [Member]
Financial liabilities:
Derivative liabilities	437,000
Future [Member] | Level 2 [Member]
Financial liabilities:
Derivative liabilities	0
Future [Member] | Level 3 [Member]
Financial liabilities:
Derivative liabilities	0
Interest Rate Swap [Member] | Total Fair Value [Member]
Financial liabilities:
Derivative liabilities - interest rate swaps - asset position	2,711,000	61,839,000
Derivative liabilities	232,013,000	365,776,000
Interest Rate Swap [Member] | Total Fair Value [Member] | Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities	2,064,105,000	2,078,601,000
Interest Rate Swap [Member] | Carrying Amount [Member]
Financial liabilities:
Derivative liabilities - interest rate swaps - asset position	2,711,000	61,839,000
Derivative liabilities	232,013,000	365,776,000
Interest Rate Swap [Member] | Carrying Amount [Member] | Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities	2,064,105,000	2,078,601,000
Interest Rate Swap [Member] | Level 1 [Member]
Financial liabilities:
Derivative liabilities - interest rate swaps - asset position	0	0
Derivative liabilities	0	0
Interest Rate Swap [Member] | Level 1 [Member] | Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities	0	0
Interest Rate Swap [Member] | Level 2 [Member]
Financial liabilities:
Derivative liabilities - interest rate swaps - asset position	2,711,000	61,839,000
Derivative liabilities	84,979,000	220,587,000
Interest Rate Swap [Member] | Level 2 [Member] | Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities	2,064,105,000	2,078,601,000
Interest Rate Swap [Member] | Level 3 [Member]
Financial liabilities:
Derivative liabilities - interest rate swaps - asset position	0	0
Derivative liabilities	147,034,000	145,189,000
Interest Rate Swap [Member] | Level 3 [Member] | Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities	0	0
Credit Derivatives [Member] | Total Fair Value [Member]
Financial liabilities:
Derivative liabilities		15,349,000
Credit Derivatives [Member] | Total Fair Value [Member] | Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities	14,496,000
Credit Derivatives [Member] | Carrying Amount [Member]
Financial liabilities:
Derivative liabilities		15,349,000
Credit Derivatives [Member] | Carrying Amount [Member] | Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities	14,496,000
Credit Derivatives [Member] | Level 1 [Member]
Financial liabilities:
Derivative liabilities		0
Credit Derivatives [Member] | Level 1 [Member] | Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities	0
Credit Derivatives [Member] | Level 2 [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]
Financial liabilities:
Derivative liabilities		$ 15,349,000
Credit Derivatives [Member] | Level 3 [Member] | Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities	$ 14,496,000