Fair Value Measurements - Summary of Carrying Amount and Fair Value of Ambac's Financial Assets and Liabilities (Detail) - Successor [Member] - USD ($)	Jun. 30, 2018	Dec. 31, 2017
Financial assets:
Fixed income securities	$ 3,993,015,000	$ 5,309,161,000
Short term investments	393,447,000	557,270,000
Other investments	394,396,000	431,630,000
Other assets	5,255,000	5,979,000
Long-term Debt	2,796,389,000	991,696,000
Variable Interest Entity, Primary Beneficiary [Member]
Financial assets:
Loans	10,751,199,000	11,529,384,000
Long-term Debt	11,454,746,000	12,160,544,000
Total Fair Value [Member]
Financial assets:
Cash	44,398,000	623,703,000
Loans	12,039,000	10,284,000
Other assets	5,255,000	5,979,000
Total financial assets	17,974,625,000	20,835,968,000
Long-term Debt		1,369,499,000
Financial liabilities:
Liabilities for net financial guarantees written	1,022,545,000	4,842,402,000
Long-term debt	3,147,361,000
Total financial liabilities	17,684,527,000	20,660,491,000
Total Fair Value [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial assets:
Restricted cash	1,052,000	978,000
Loans	10,751,199,000	11,529,384,000
Financial liabilities:
Long-term debt	11,454,746,000	12,160,544,000
Total Fair Value [Member] | Future [Member]
Financial assets:
Derivative assets	524,000
Reported Value Measurement [Member]
Financial assets:
Cash	44,398,000	623,703,000
Loans	10,007,000	10,358,000
Other assets	5,255,000	5,979,000
Total financial assets	17,988,518,000	20,853,695,000
Long-term Debt		1,428,680,000
Financial liabilities:
Liabilities for net financial guarantees written	(463,146,000)	3,435,438,000
Long-term debt	3,033,947,000
Total financial liabilities	16,085,422,000	19,312,708,000
Reported Value Measurement [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial assets:
Restricted cash	1,052,000	978,000
Loans	10,751,199,000	11,529,384,000
Financial liabilities:
Long-term debt	11,454,746,000	12,160,544,000
Reported Value Measurement [Member] | Future [Member]
Financial assets:
Derivative assets	524,000
Other Assets [Member] | Total Fair Value [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	56,147,000	73,199,000
Other Assets [Member] | Reported Value Measurement [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	56,147,000	73,199,000
Interest Rate Swaps - Liability Position [Member] | Total Fair Value [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets - interest rate swaps - liability position	(161,000)
Interest Rate Swaps - Liability Position [Member] | Total Fair Value [Member] | Currency Swaps [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial assets:
Derivative assets	60,403,000	54,877,000
Interest Rate Swaps - Liability Position [Member] | Reported Value Measurement [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets - interest rate swaps - liability position	(161,000)
Interest Rate Swaps - Liability Position [Member] | Reported Value Measurement [Member] | Currency Swaps [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial assets:
Derivative assets	60,403,000	54,877,000
Level 1 [Member]
Financial assets:
Cash	23,993,000	615,073,000
Loans	0	0
Other assets	0	0
Total financial assets	506,682,000	1,173,321,000
Long-term Debt		0
Financial liabilities:
Liabilities for net financial guarantees written	0	0
Long-term debt	0
Total financial liabilities	0	1,348,000
Level 1 [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial assets:
Restricted cash	1,052,000	978,000
Loans	0	0
Financial liabilities:
Long-term debt	0	0
Level 1 [Member] | Future [Member]
Financial assets:
Derivative assets	524,000
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] | Interest Rate Swap [Member]
Financial assets:
Derivative assets - interest rate swaps - liability position	0
Level 1 [Member] | Interest Rate Swaps - Liability Position [Member] | Currency Swaps [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial assets:
Derivative assets	0	0
Level 2 [Member]
Financial assets:
Cash	20,405,000	8,630,000
Loans	0	0
Other assets	0	0
Total financial assets	3,508,148,000	4,005,195,000
Long-term Debt		1,046,511,000
Financial liabilities:
Liabilities for net financial guarantees written	0	0
Long-term debt	2,733,359,000
Total financial liabilities	13,729,016,000	12,735,499,000
Level 2 [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial assets:
Restricted cash	0	0
Loans	0	0
Financial liabilities:
Long-term debt	8,937,108,000	9,402,856,000
Level 2 [Member] | Future [Member]
Financial assets:
Derivative assets	0
Level 2 [Member] | Other Assets [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	9,208,000	11,825,000
Level 2 [Member] | Interest Rate Swaps - Liability Position [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets - interest rate swaps - liability position	(161,000)
Level 2 [Member] | Interest Rate Swaps - Liability Position [Member] | Currency Swaps [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial assets:
Derivative assets	60,403,000	54,877,000
Level 3 [Member]
Financial assets:
Cash	0	0
Loans	12,039,000	10,284,000
Other assets	5,255,000	5,979,000
Total financial assets	13,662,729,000	15,347,011,000
Long-term Debt		322,988,000
Financial liabilities:
Liabilities for net financial guarantees written	1,022,545,000	4,842,402,000
Long-term debt	414,002,000
Total financial liabilities	3,955,511,000	7,923,644,000
Level 3 [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial assets:
Restricted cash	0	0
Loans	10,751,199,000	11,529,384,000
Financial liabilities:
Long-term debt	2,517,638,000	2,757,688,000
Level 3 [Member] | Future [Member]
Financial assets:
Derivative assets	0
Level 3 [Member] | Other Assets [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	46,939,000	61,374,000
Level 3 [Member] | Interest Rate Swaps - Liability Position [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets - interest rate swaps - liability position	0
Level 3 [Member] | Interest Rate Swaps - Liability Position [Member] | Currency Swaps [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial assets:
Derivative assets	0	0
Municipal Bonds [Member] | Total Fair Value [Member]
Financial assets:
Fixed income securities	899,122,000	779,834,000
Municipal Bonds [Member] | Reported Value Measurement [Member]
Financial assets:
Fixed income securities	899,122,000	779,834,000
Municipal Bonds [Member] | Level 1 [Member]
Financial assets:
Fixed income securities	0	0
Municipal Bonds [Member] | Level 2 [Member]
Financial assets:
Fixed income securities	899,122,000	779,834,000
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	1,350,787,000	860,075,000
Corporate Obligations [Member] | Total Fair Value [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial assets:
Fixed income securities	2,756,924,000	2,914,145,000
Corporate Obligations [Member] | Reported Value Measurement [Member]
Financial assets:
Fixed income securities	1,350,787,000	860,075,000
Corporate Obligations [Member] | Reported Value Measurement [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial assets:
Fixed income securities	2,756,924,000	2,914,145,000
Corporate Obligations [Member] | Level 1 [Member]
Financial assets:
Fixed income securities	450,000	450,000
Corporate Obligations [Member] | Level 1 [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial assets:
Fixed income securities	0	0
Corporate Obligations [Member] | Level 2 [Member]
Financial assets:
Fixed income securities	1,350,337,000	859,625,000
Corporate Obligations [Member] | Level 2 [Member] | Variable Interest Entity, Primary Beneficiary [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, Primary Beneficiary [Member]
Financial assets:
Fixed income securities	2,756,924,000	2,914,145,000
Foreign Obligations [Member] | Total Fair Value [Member]
Financial assets:
Fixed income securities	25,442,000	26,543,000
Foreign Obligations [Member] | Reported Value Measurement [Member]
Financial assets:
Fixed income securities	25,442,000	26,543,000
Foreign Obligations [Member] | Level 1 [Member]
Financial assets:
Fixed income securities	24,346,000	25,615,000
Foreign Obligations [Member] | Level 2 [Member]
Financial assets:
Fixed income securities	1,096,000	928,000
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	82,413,000	85,408,000
Fixed income securities, pledged as collateral	84,641,000	99,719,000
US Government Debt Securities [Member] | Reported Value Measurement [Member]
Financial assets:
Fixed income securities	82,413,000	85,408,000
Fixed income securities, pledged as collateral	84,641,000	99,719,000
US Government Debt Securities [Member] | Level 1 [Member]
Financial assets:
Fixed income securities	82,413,000	85,408,000
Fixed income securities, pledged as collateral	84,641,000	99,719,000
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
Residential Mortgage-Backed Securities [Member] | Total Fair Value [Member]
Financial assets:
Fixed income securities	512,309,000	2,251,333,000
Residential Mortgage-Backed Securities [Member] | Reported Value Measurement [Member]
Financial assets:
Fixed income securities	512,309,000	2,251,333,000
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	512,309,000	1,515,316,000
Residential Mortgage-Backed Securities [Member] | Level 3 [Member]
Financial assets:
Fixed income securities	0	736,017,000
Collateralized Debt Obligations [Member] | Total Fair Value [Member]
Financial assets:
Fixed income securities	65,457,000	51,037,000
Collateralized Debt Obligations [Member] | Reported Value Measurement [Member]
Financial assets:
Fixed income securities	65,457,000	51,037,000
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	65,457,000	51,037,000
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	579,397,000	597,942,000
Asset-backed Securities [Member] | Reported Value Measurement [Member]
Financial assets:
Fixed income securities	579,397,000	597,942,000
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	510,595,000	525,402,000
Asset-backed Securities [Member] | Level 3 [Member]
Financial assets:
Fixed income securities	68,802,000	72,540,000
Short-Term [Member] | Total Fair Value [Member]
Financial assets:
Short term investments	393,447,000	557,270,000
Short-Term [Member] | Reported Value Measurement [Member]
Financial assets:
Short term investments	393,447,000	557,270,000
Short-Term [Member] | Level 1 [Member]
Financial assets:
Short term investments	314,070,000	389,299,000
Short-Term [Member] | Level 2 [Member]
Financial assets:
Short term investments	79,377,000	167,971,000
Short-Term [Member] | Level 3 [Member]
Financial assets:
Short term investments	0	0
Other [Member] | Total Fair Value [Member]
Financial assets:
Other investments	378,471,000	413,977,000
Other [Member] | Reported Value Measurement [Member]
Financial assets:
Other investments	394,396,000	431,630,000
Other [Member] | Level 1 [Member]
Financial assets:
Other investments	59,834,000	56,498,000
Other [Member] | Level 2 [Member]
Financial assets:
Other investments	0	29,750,000
Other [Member] | Level 3 [Member]
Financial assets:
Other investments	21,571,000	17,288,000
Future [Member] | Total Fair Value [Member] | Future [Member]
Financial liabilities:
Derivative liabilities		1,348,000
Future [Member] | Reported Value Measurement [Member] | Future [Member]
Financial liabilities:
Derivative liabilities		1,348,000
Future [Member] | Level 1 [Member] | Future [Member]
Financial liabilities:
Derivative liabilities		1,348,000
Future [Member] | Level 2 [Member] | Future [Member]
Financial liabilities:
Derivative liabilities		0
Future [Member] | Level 3 [Member] | Future [Member]
Financial liabilities:
Derivative liabilities		0
Interest Rate Swap [Member] | Total Fair Value [Member]
Financial liabilities:
Derivative liabilities - interest rate swaps - asset position	0
Derivative liabilities	66,322,000
Interest Rate Swap [Member] | Total Fair Value [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial liabilities:
Derivative liabilities	1,992,227,000
Interest Rate Swap [Member] | Reported Value Measurement [Member]
Financial liabilities:
Derivative liabilities - interest rate swaps - asset position	0
Derivative liabilities	66,322,000
Interest Rate Swap [Member] | Reported Value Measurement [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial liabilities:
Derivative liabilities	1,992,227,000
Interest Rate Swap [Member] | Other Assets [Member] | Total Fair Value [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities - interest rate swaps - asset position		(627,000)
Interest Rate Swap [Member] | Other Assets [Member] | Reported Value Measurement [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities - interest rate swaps - asset position		(627,000)
Interest Rate Swap [Member] | Interest Rate Swaps - Liability Position [Member] | Total Fair Value [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities		81,495,000
Interest Rate Swap [Member] | Interest Rate Swaps - Liability Position [Member] | Total Fair Value [Member] | Interest Rate Swap [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial liabilities:
Derivative liabilities		2,205,264,000
Interest Rate Swap [Member] | Interest Rate Swaps - Liability Position [Member] | Reported Value Measurement [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities		81,495,000
Interest Rate Swap [Member] | Interest Rate Swaps - Liability Position [Member] | Reported Value Measurement [Member] | Interest Rate Swap [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial liabilities:
Derivative liabilities		2,205,264,000
Interest Rate Swap [Member] | Level 1 [Member]
Financial liabilities:
Derivative liabilities - interest rate swaps - asset position	0
Derivative liabilities	0
Interest Rate Swap [Member] | Level 1 [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial liabilities:
Derivative liabilities	0
Interest Rate Swap [Member] | Level 1 [Member] | Other Assets [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities - interest rate swaps - asset position		0
Interest Rate Swap [Member] | Level 1 [Member] | Interest Rate Swaps - Liability Position [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities		0
Interest Rate Swap [Member] | Level 1 [Member] | Interest Rate Swaps - Liability Position [Member] | Interest Rate Swap [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial liabilities:
Derivative liabilities		0
Interest Rate Swap [Member] | Level 2 [Member]
Financial liabilities:
Derivative liabilities - interest rate swaps - asset position
Derivative liabilities	66,322,000
Interest Rate Swap [Member] | Level 2 [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial liabilities:
Derivative liabilities	1,992,227,000
Interest Rate Swap [Member] | Level 2 [Member] | Other Assets [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities - interest rate swaps - asset position		(627,000)
Interest Rate Swap [Member] | Level 2 [Member] | Interest Rate Swaps - Liability Position [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities		81,495,000
Interest Rate Swap [Member] | Level 2 [Member] | Interest Rate Swaps - Liability Position [Member] | Interest Rate Swap [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial liabilities:
Derivative liabilities		2,205,264,000
Interest Rate Swap [Member] | Level 3 [Member]
Financial liabilities:
Derivative liabilities - interest rate swaps - asset position	0
Derivative liabilities	0
Interest Rate Swap [Member] | Level 3 [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial liabilities:
Derivative liabilities	0
Interest Rate Swap [Member] | Level 3 [Member] | Other Assets [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities - interest rate swaps - asset position		0
Interest Rate Swap [Member] | Level 3 [Member] | Interest Rate Swaps - Liability Position [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities		0
Interest Rate Swap [Member] | Level 3 [Member] | Interest Rate Swaps - Liability Position [Member] | Interest Rate Swap [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial liabilities:
Derivative liabilities		0
Credit Derivatives [Member] | Total Fair Value [Member] | Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities	1,326,000	566,000
Credit Derivatives [Member] | Reported Value Measurement [Member] | Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities	1,326,000	566,000
Credit Derivatives [Member] | Level 1 [Member] | Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities	0	0
Credit Derivatives [Member] | Level 2 [Member] | Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities	0	0
Credit Derivatives [Member] | Level 3 [Member] | Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities	1,326,000	566,000
Investments Measured at NAV as a Practical Expedient [Member]
Financial assets:
Other investments	$ 297,066,000	$ 310,441,000