Fair Value Measurements - Summary of Carrying Amount and Fair Value of Ambac's Financial Assets and Liabilities (Detail) - Successor [Member] - USD ($)	Jun. 30, 2017	Dec. 31, 2016
Financial assets:
Fixed income securities	$ 5,887,129,000	$ 6,049,908,000
Short term investments	504,006,000	430,788,000
Other investments	452,032,000	450,307,000
Other assets	6,691,000	7,382,000
Borrowings under Guaranteed Investment Agreements	0	82,358,000
Variable Interest Entity [Member]
Financial assets:
Loans	11,301,298,000	10,658,963,000
Total Fair Value [Member]
Financial assets:
Cash	54,238,000	91,025,000
Loans	10,170,000	4,066,000
Other assets	6,691,000	7,382,000
Total financial assets	20,567,598,000	20,032,169,000
Financial liabilities:
Obligations under investment and repurchase agreements		82,333,000
Liabilities for net financial guarantees written	5,214,437,000	4,490,070,000
Long-term debt	1,371,102,000	1,494,340,000
Total financial liabilities	20,643,841,000	19,620,566,000
Total Fair Value [Member] | Variable Interest Entity [Member]
Financial assets:
Restricted cash	5,015,000	4,873,000
Loans	11,301,298,000	10,658,963,000
Financial liabilities:
Long-term debt	11,902,682,000	11,155,936,000
Total Fair Value [Member] | Future [Member]
Financial assets:
Derivative assets	3,111,000	536,000
Reported Value Measurement [Member]
Financial assets:
Cash	54,238,000	91,025,000
Loans	10,125,000	4,160,000
Other assets	6,691,000	7,382,000
Total financial assets	20,583,398,000	20,047,333,000
Borrowings under Guaranteed Investment Agreements		82,358,000
Financial liabilities:
Liabilities for net financial guarantees written	3,212,581,000	3,009,943,000
Long-term debt	1,386,124,000	1,536,352,000
Total financial liabilities	18,657,007,000	18,182,476,000
Reported Value Measurement [Member] | Variable Interest Entity [Member]
Financial assets:
Restricted cash	5,015,000	4,873,000
Loans	11,301,298,000	10,658,963,000
Financial liabilities:
Long-term debt	11,902,682,000	11,155,936,000
Reported Value Measurement [Member] | Future [Member]
Financial assets:
Derivative assets	3,111,000	536,000
Other Assets [Member] | Total Fair Value [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	75,936,000	77,206,000
Other Assets [Member] | Reported Value Measurement [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	75,936,000	77,206,000
Interest Rate Swaps - Liability Position [Member] | Total Fair Value [Member] | Currency Swaps [Member] | Variable Interest Entity [Member]
Financial assets:
Derivative assets	65,507,000	80,407,000
Interest Rate Swaps - Liability Position [Member] | Reported Value Measurement [Member] | Currency Swaps [Member] | Variable Interest Entity [Member]
Financial assets:
Derivative assets	65,507,000	80,407,000
Level 1 [Member]
Financial assets:
Cash	26,300,000	46,587,000
Loans	0	0
Other assets	0	0
Total financial assets	661,838,000	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	0	0
Level 1 [Member] | Variable Interest Entity [Member]
Financial assets:
Restricted cash	5,015,000	4,873,000
Loans	0	0
Financial liabilities:
Long-term debt	0	0
Level 1 [Member] | Future [Member]
Financial assets:
Derivative assets	3,111,000	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	27,938,000	44,438,000
Loans	0	0
Other assets	0	0
Total financial assets	4,691,434,000	4,914,330,000
Financial liabilities:
Obligations under investment and repurchase agreements		0
Liabilities for net financial guarantees written	0	0
Long-term debt	1,042,366,000	1,147,728,000
Total financial liabilities	12,287,444,000	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	9,098,464,000	8,573,716,000
Level 2 [Member] | Future [Member]
Financial assets:
Derivative assets	0	0
Level 2 [Member] | Other Assets [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	14,201,000	16,950,000
Level 2 [Member] | Interest Rate Swaps - Liability Position [Member] | Currency Swaps [Member] | Variable Interest Entity [Member]
Financial assets:
Derivative assets	65,507,000	80,407,000
Level 3 [Member]
Financial assets:
Cash	0	0
Loans	10,170,000	4,066,000
Other assets	6,691,000	7,382,000
Total financial assets	14,884,465,000	14,130,870,000
Financial liabilities:
Obligations under investment and repurchase agreements		82,333,000
Liabilities for net financial guarantees written	5,214,437,000	4,490,070,000
Long-term debt	328,736,000	346,612,000
Total financial liabilities	8,356,397,000	7,661,773,000
Level 3 [Member] | Variable Interest Entity [Member]
Financial assets:
Restricted cash	0	0
Loans	11,301,298,000	10,658,963,000
Financial liabilities:
Long-term debt	2,804,218,000	2,582,220,000
Level 3 [Member] | Future [Member]
Financial assets:
Derivative assets	0	0
Level 3 [Member] | Other Assets [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	61,735,000	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	661,506,000	374,368,000
Municipal Bonds [Member] | Reported Value Measurement [Member]
Financial assets:
Fixed income securities	661,506,000	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	661,506,000	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	1,500,413,000	1,802,165,000
Corporate Obligations [Member] | Total Fair Value [Member] | Variable Interest Entity [Member]
Financial assets:
Fixed income securities	2,722,316,000	2,622,566,000
Corporate Obligations [Member] | Reported Value Measurement [Member]
Financial assets:
Fixed income securities	1,500,413,000	1,802,165,000
Corporate Obligations [Member] | Reported Value Measurement [Member] | Variable Interest Entity [Member]
Financial assets:
Fixed income securities	2,722,316,000	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	1,500,413,000	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	2,722,316,000	2,622,566,000
Foreign Obligations [Member] | Total Fair Value [Member]
Financial assets:
Fixed income securities	29,746,000	43,135,000
Foreign Obligations [Member] | Reported Value Measurement [Member]
Financial assets:
Fixed income securities	29,746,000	43,135,000
Foreign Obligations [Member] | Level 1 [Member]
Financial assets:
Fixed income securities	28,802,000	42,212,000
Foreign Obligations [Member] | Level 2 [Member]
Financial assets:
Fixed income securities	944,000	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	41,633,000	36,186,000
Fixed income securities, pledged as collateral	64,928,000	64,905,000
US Government Debt Securities [Member] | Reported Value Measurement [Member]
Financial assets:
Fixed income securities	41,633,000	36,186,000
Fixed income securities, pledged as collateral	64,928,000	64,905,000
US Government Debt Securities [Member] | Level 1 [Member]
Financial assets:
Fixed income securities	41,633,000	36,186,000
Fixed income securities, pledged as collateral	64,928,000	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		4,060,000
U.S. Agency Obligations [Member] | Reported Value Measurement [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] | Total Fair Value [Member]
Financial assets:
Fixed income securities	2,299,528,000	2,351,595,000
Residential Mortgage-Backed Securities [Member] | Reported Value Measurement [Member]
Financial assets:
Fixed income securities	2,299,528,000	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	1,599,212,000	1,654,882,000
Residential Mortgage-Backed Securities [Member] | Level 3 [Member]
Financial assets:
Fixed income securities	700,316,000	696,713,000
Collateralized Debt Obligations [Member] | Total Fair Value [Member]
Financial assets:
Fixed income securities	125,106,000	113,923,000
Collateralized Debt Obligations [Member] | Reported Value Measurement [Member]
Financial assets:
Fixed income securities	125,106,000	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	125,106,000	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	660,263,000	828,783,000
Asset-backed Securities [Member] | Reported Value Measurement [Member]
Financial assets:
Fixed income securities	660,263,000	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	594,897,000	762,793,000
Asset-backed Securities [Member] | Level 3 [Member]
Financial assets:
Fixed income securities	65,366,000	65,990,000
Short-Term [Member] | Total Fair Value [Member]
Financial assets:
Short term investments	504,006,000	430,788,000
Short-Term [Member] | Reported Value Measurement [Member]
Financial assets:
Short term investments	504,006,000	430,788,000
Short-Term [Member] | Level 1 [Member]
Financial assets:
Short term investments	402,296,000	371,367,000
Short-Term [Member] | Level 2 [Member]
Financial assets:
Short term investments	101,710,000	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	436,187,000	435,237,000
Other [Member] | Reported Value Measurement [Member]
Financial assets:
Other investments	452,032,000	450,307,000
Other [Member] | Level 1 [Member]
Financial assets:
Other investments	89,753,000	83,791,000
Other [Member] | Level 2 [Member]
Financial assets:
Other investments	0	0
Other [Member] | Level 3 [Member]
Financial assets:
Other investments	16,573,000	14,934,000
Future [Member] | Total Fair Value [Member]
Financial liabilities:
Derivative liabilities	0
Future [Member] | Reported Value Measurement [Member]
Financial liabilities:
Derivative liabilities	0
Future [Member] | Level 1 [Member]
Financial liabilities:
Derivative liabilities	0
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	731,000	61,839,000
Derivative liabilities	83,274,000	365,776,000
Interest Rate Swap [Member] | Total Fair Value [Member] | Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities	2,064,071,000	2,078,601,000
Interest Rate Swap [Member] | Reported Value Measurement [Member]
Financial liabilities:
Derivative liabilities - interest rate swaps - asset position	731,000	61,839,000
Derivative liabilities	83,274,000	365,776,000
Interest Rate Swap [Member] | Reported Value Measurement [Member] | Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities	2,064,071,000	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	731,000	61,839,000
Derivative liabilities	83,274,000	220,587,000
Interest Rate Swap [Member] | Level 2 [Member] | Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities	2,064,071,000	2,078,601,000
Interest Rate Swap [Member] | Level 3 [Member]
Financial liabilities:
Derivative liabilities - interest rate swaps - asset position	0	0
Derivative liabilities	0	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	9,006,000
Credit Derivatives [Member] | Reported Value Measurement [Member]
Financial liabilities:
Derivative liabilities		15,349,000
Credit Derivatives [Member] | Reported Value Measurement [Member] | Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities	9,006,000
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	9,006,000
Investments Measured at NAV as a Practical Expedient [Member]
Financial assets:
Other investments	$ 329,861,000	$ 336,513,000