Fair Value Measurements - Summary of Carrying Amount and Fair Value of Ambac's Financial Assets and Liabilities (Detail) - Successor [Member] - USD ($)	Sep. 30, 2017	Dec. 31, 2016
Financial assets:
Fixed income securities	$ 5,794,058,000	$ 6,049,908,000
Short term investments	716,516,000	430,788,000
Other investments	439,987,000	450,307,000
Other assets	6,337,000	7,382,000
Borrowings under Guaranteed Investment Agreements	0	82,358,000
Variable Interest Entity [Member]
Financial assets:
Loans	11,557,788,000	10,658,963,000
Total Fair Value [Member]
Financial assets:
Cash	107,018,000	91,025,000
Loans	10,370,000	4,066,000
Other assets	6,337,000	7,382,000
Total financial assets	20,857,944,000	20,032,169,000
Financial liabilities:
Obligations under investment and repurchase agreements		82,333,000
Liabilities for net financial guarantees written	5,187,773,000	4,490,070,000
Long-term debt	1,392,295,000	1,494,340,000
Total financial liabilities	20,989,458,000	19,620,566,000
Total Fair Value [Member] | Variable Interest Entity [Member]
Financial assets:
Restricted cash	37,793,000	4,873,000
Loans	11,557,788,000	10,658,963,000
Financial liabilities:
Long-term debt	12,229,569,000	11,155,936,000
Total Fair Value [Member] | Future [Member]
Financial assets:
Derivative assets	2,236,000	536,000
Reported Value Measurement [Member]
Financial assets:
Cash	107,018,000	91,025,000
Loans	10,390,000	4,160,000
Other assets	6,337,000	7,382,000
Total financial assets	20,873,980,000	20,047,333,000
Borrowings under Guaranteed Investment Agreements		82,358,000
Financial liabilities:
Liabilities for net financial guarantees written	3,302,677,000	3,009,943,000
Long-term debt	1,405,670,000	1,536,352,000
Total financial liabilities	19,117,737,000	18,182,476,000
Reported Value Measurement [Member] | Variable Interest Entity [Member]
Financial assets:
Restricted cash	37,793,000	4,873,000
Loans	11,557,788,000	10,658,963,000
Financial liabilities:
Long-term debt	12,229,569,000	11,155,936,000
Reported Value Measurement [Member] | Future [Member]
Financial assets:
Derivative assets	2,236,000	536,000
Other Assets [Member] | Total Fair Value [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	75,051,000	77,206,000
Other Assets [Member] | Reported Value Measurement [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	75,051,000	77,206,000
Interest Rate Swaps - Liability Position [Member] | Total Fair Value [Member] | Currency Swaps [Member] | Variable Interest Entity [Member]
Financial assets:
Derivative assets	57,714,000	80,407,000
Interest Rate Swaps - Liability Position [Member] | Reported Value Measurement [Member] | Currency Swaps [Member] | Variable Interest Entity [Member]
Financial assets:
Derivative assets	57,714,000	80,407,000
Level 1 [Member]
Financial assets:
Cash	63,115,000	46,587,000
Loans	0	0
Other assets	0	0
Total financial assets	802,662,000	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	37,793,000	4,873,000
Loans	0	0
Financial liabilities:
Long-term debt	0	0
Level 1 [Member] | Future [Member]
Financial assets:
Derivative assets	2,236,000	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	43,903,000	44,438,000
Loans	0	0
Other assets	0	0
Total financial assets	4,541,717,000	4,914,330,000
Financial liabilities:
Obligations under investment and repurchase agreements		0
Liabilities for net financial guarantees written	0	0
Long-term debt	1,053,453,000	1,147,728,000
Total financial liabilities	12,507,498,000	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	9,283,185,000	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	13,001,000	16,950,000
Level 2 [Member] | Interest Rate Swaps - Liability Position [Member] | Currency Swaps [Member] | Variable Interest Entity [Member]
Financial assets:
Derivative assets	57,714,000	80,407,000
Level 3 [Member]
Financial assets:
Cash	0	0
Loans	10,370,000	4,066,000
Other assets	6,337,000	7,382,000
Total financial assets	15,215,623,000	14,130,870,000
Financial liabilities:
Obligations under investment and repurchase agreements		82,333,000
Liabilities for net financial guarantees written	5,187,773,000	4,490,070,000
Long-term debt	338,842,000	346,612,000
Total financial liabilities	8,481,960,000	7,661,773,000
Level 3 [Member] | Variable Interest Entity [Member]
Financial assets:
Restricted cash	0	0
Loans	11,557,788,000	10,658,963,000
Financial liabilities:
Long-term debt	2,946,384,000	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	62,050,000	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	539,254,000	374,368,000
Municipal Bonds [Member] | Reported Value Measurement [Member]
Financial assets:
Fixed income securities	539,254,000	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	539,254,000	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	1,285,468,000	1,802,165,000
Corporate Obligations [Member] | Total Fair Value [Member] | Variable Interest Entity [Member]
Financial assets:
Fixed income securities	2,785,608,000	2,622,566,000
Corporate Obligations [Member] | Reported Value Measurement [Member]
Financial assets:
Fixed income securities	1,285,468,000	1,802,165,000
Corporate Obligations [Member] | Reported Value Measurement [Member] | Variable Interest Entity [Member]
Financial assets:
Fixed income securities	2,785,608,000	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	1,285,468,000	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	2,785,608,000	2,622,566,000
Foreign Obligations [Member] | Total Fair Value [Member]
Financial assets:
Fixed income securities	30,210,000	43,135,000
Foreign Obligations [Member] | Reported Value Measurement [Member]
Financial assets:
Fixed income securities	30,210,000	43,135,000
Foreign Obligations [Member] | Level 1 [Member]
Financial assets:
Fixed income securities	29,271,000	42,212,000
Foreign Obligations [Member] | Level 2 [Member]
Financial assets:
Fixed income securities	939,000	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	89,824,000	36,186,000
Fixed income securities, pledged as collateral		64,905,000
US Government Debt Securities [Member] | Reported Value Measurement [Member]
Financial assets:
Fixed income securities	89,824,000	36,186,000
Fixed income securities, pledged as collateral		64,905,000
US Government Debt Securities [Member] | Level 1 [Member]
Financial assets:
Fixed income securities	89,824,000	36,186,000
Fixed income securities, pledged as collateral		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
US Government Debt Securities [Member] | Level 3 [Member]
Financial assets:
Fixed income securities	0	0
Fixed income securities, pledged as collateral		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	2,279,639,000	2,351,595,000
Residential Mortgage-Backed Securities [Member] | Reported Value Measurement [Member]
Financial assets:
Fixed income securities	2,279,639,000	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	1,568,968,000	1,654,882,000
Residential Mortgage-Backed Securities [Member] | Level 3 [Member]
Financial assets:
Fixed income securities	710,671,000	696,713,000
Collateralized Debt Obligations [Member] | Total Fair Value [Member]
Financial assets:
Fixed income securities	90,925,000	113,923,000
Collateralized Debt Obligations [Member] | Reported Value Measurement [Member]
Financial assets:
Fixed income securities	90,925,000	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	90,925,000	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	662,798,000	828,783,000
Asset-backed Securities [Member] | Reported Value Measurement [Member]
Financial assets:
Fixed income securities	662,798,000	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	597,660,000	762,793,000
Asset-backed Securities [Member] | Level 3 [Member]
Financial assets:
Fixed income securities	65,138,000	65,990,000
Short-Term [Member] | Total Fair Value [Member]
Financial assets:
Fixed income securities, pledged as collateral	99,424,000
Short term investments	716,516,000	430,788,000
Short-Term [Member] | Reported Value Measurement [Member]
Financial assets:
Fixed income securities, pledged as collateral	99,424,000
Short term investments	716,516,000	430,788,000
Short-Term [Member] | Level 1 [Member]
Financial assets:
Fixed income securities, pledged as collateral	99,424,000
Short term investments	412,681,000	371,367,000
Short-Term [Member] | Level 2 [Member]
Financial assets:
Fixed income securities, pledged as collateral	0
Short term investments	303,835,000	59,421,000
Short-Term [Member] | Level 3 [Member]
Financial assets:
Fixed income securities, pledged as collateral	0
Short term investments	0	0
Other [Member] | Total Fair Value [Member]
Financial assets:
Other investments	423,971,000	435,237,000
Other [Member] | Reported Value Measurement [Member]
Financial assets:
Other investments	439,987,000	450,307,000
Other [Member] | Level 1 [Member]
Financial assets:
Other investments	68,318,000	83,791,000
Other [Member] | Level 2 [Member]
Financial assets:
Other investments	40,050,000	0
Other [Member] | Level 3 [Member]
Financial assets:
Other investments	17,661,000	14,934,000
Interest Rate Swap [Member] | Total Fair Value [Member]
Financial liabilities:
Derivative liabilities - interest rate swaps - asset position	1,152,000	61,839,000
Derivative liabilities	83,090,000	365,776,000
Interest Rate Swap [Member] | Total Fair Value [Member] | Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities	2,088,922,000	2,078,601,000
Interest Rate Swap [Member] | Reported Value Measurement [Member]
Financial liabilities:
Derivative liabilities - interest rate swaps - asset position	1,152,000	61,839,000
Derivative liabilities	83,090,000	365,776,000
Interest Rate Swap [Member] | Reported Value Measurement [Member] | Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities	2,088,922,000	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	1,152,000	61,839,000
Derivative liabilities	83,090,000	220,587,000
Interest Rate Swap [Member] | Level 2 [Member] | Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities	2,088,922,000	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	8,961,000
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	8,961,000
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	8,961,000
Investments Measured at NAV as a Practical Expedient [Member]
Financial assets:
Other investments	$ 297,942,000	$ 336,513,000