Fair Value Measurements - Summary of Carrying Amount and Fair Value of Ambac's Financial Assets and Liabilities (Detail) - Successor [Member] - USD ($)	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Financial assets:
Fixed income securities	$ 4,277,297,000	$ 5,309,161,000
Short term investments	321,119,000	557,270,000
Other investments	419,896,000	431,630,000
Other assets	5,621,000	5,979,000
Long-term Debt	2,957,732,000	991,696,000
Variable Interest Entity, Primary Beneficiary [Member]
Financial assets:
Loans	11,558,331,000	11,529,384,000
Long-term Debt	12,270,124,000	12,160,544,000
Total Fair Value [Member]
Financial assets:
Cash	38,485,000	623,703,000
Loans	12,042,000	10,284,000
Other assets	5,621,000	5,979,000
Total financial assets	19,272,961,000	20,835,968,000
Long-term Debt		1,369,499,000
Financial liabilities:
Liabilities for net financial guarantees written	900,880,000		$ 4,842,402,000
Long-term debt	3,265,029,000
Total financial liabilities	18,663,073,000		20,660,491,000
Total Fair Value [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial assets:
Restricted cash	1,134,000	978,000
Loans	11,558,331,000	11,529,384,000
Financial liabilities:
Long-term debt	12,270,124,000		12,160,544,000
Reported Value Measurement [Member]
Financial assets:
Cash	38,485,000	623,703,000
Loans	10,643,000	10,358,000
Other assets	5,621,000	5,979,000
Total financial assets	19,289,386,000	20,853,695,000
Long-term Debt		1,428,680,000
Financial liabilities:
Liabilities for net financial guarantees written	(432,722,000)		3,435,438,000
Long-term debt	3,201,931,000
Total financial liabilities	17,266,373,000		19,312,708,000
Reported Value Measurement [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial assets:
Restricted cash	1,134,000	978,000
Loans	11,558,331,000	11,529,384,000
Financial liabilities:
Long-term debt	12,270,124,000		12,160,544,000
Other Assets [Member] | Total Fair Value [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	60,198,000	73,199,000
Other Assets [Member] | Reported Value Measurement [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	60,198,000	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	(2,000)
Interest Rate Swaps - Liability Position [Member] | Total Fair Value [Member] | Currency Swaps [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial assets:
Derivative assets	46,260,000	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	(2,000)
Interest Rate Swaps - Liability Position [Member] | Reported Value Measurement [Member] | Currency Swaps [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial assets:
Derivative assets	46,260,000	54,877,000
Level 1 [Member]
Financial assets:
Cash	35,033,000	615,073,000
Loans	0	0
Other assets	0	0
Total financial assets	476,076,000	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	1,134,000	978,000
Loans	0	0
Financial liabilities:
Long-term debt	0		0
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	3,452,000	8,630,000
Loans	0	0
Other assets	0	0
Total financial assets	3,792,035,000	4,005,195,000
Long-term Debt		1,046,511,000
Financial liabilities:
Liabilities for net financial guarantees written	0		0
Long-term debt	2,651,952,000
Total financial liabilities	14,451,581,000		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	9,573,607,000		9,402,856,000
Level 2 [Member] | Other Assets [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	9,272,000	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	(2,000)
Level 2 [Member] | Interest Rate Swaps - Liability Position [Member] | Currency Swaps [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial assets:
Derivative assets	46,260,000	54,877,000
Level 3 [Member]
Financial assets:
Cash	0	0
Loans	12,042,000	10,284,000
Other assets	5,621,000	5,979,000
Total financial assets	14,678,698,000	15,347,011,000
Long-term Debt		322,988,000
Financial liabilities:
Liabilities for net financial guarantees written	900,880,000		4,842,402,000
Long-term debt	613,077,000
Total financial liabilities	4,211,492,000		7,923,644,000
Level 3 [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial assets:
Restricted cash	0	0
Loans	11,558,331,000	11,529,384,000
Financial liabilities:
Long-term debt	2,696,517,000		2,757,688,000
Level 3 [Member] | Other Assets [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	50,926,000	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	855,387,000	779,834,000
Municipal Bonds [Member] | Reported Value Measurement [Member]
Financial assets:
Fixed income securities	855,387,000	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	855,387,000	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	1,438,993,000	860,075,000
Corporate Obligations [Member] | Total Fair Value [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial assets:
Fixed income securities	2,955,763,000	2,914,145,000
Corporate Obligations [Member] | Reported Value Measurement [Member]
Financial assets:
Fixed income securities	1,438,993,000	860,075,000
Corporate Obligations [Member] | Reported Value Measurement [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial assets:
Fixed income securities	2,955,763,000	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	1,438,543,000	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	2,955,763,000	2,914,145,000
Foreign Obligations [Member] | Total Fair Value [Member]
Financial assets:
Fixed income securities	26,395,000	26,543,000
Foreign Obligations [Member] | Reported Value Measurement [Member]
Financial assets:
Fixed income securities	26,395,000	26,543,000
Foreign Obligations [Member] | Level 1 [Member]
Financial assets:
Fixed income securities	25,492,000	25,615,000
Foreign Obligations [Member] | Level 2 [Member]
Financial assets:
Fixed income securities	903,000	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	87,261,000	85,408,000
Fixed income securities, pledged as collateral	84,240,000	99,719,000
US Government Debt Securities [Member] | Reported Value Measurement [Member]
Financial assets:
Fixed income securities	87,261,000	85,408,000
Fixed income securities, pledged as collateral	84,240,000	99,719,000
US Government Debt Securities [Member] | Level 1 [Member]
Financial assets:
Fixed income securities	87,261,000	85,408,000
Fixed income securities, pledged as collateral	84,240,000	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	823,296,000	2,251,333,000
Residential Mortgage-Backed Securities [Member] | Reported Value Measurement [Member]
Financial assets:
Fixed income securities	823,296,000	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	817,987,000	1,515,316,000
Residential Mortgage-Backed Securities [Member] | Level 3 [Member]
Financial assets:
Fixed income securities	5,309,000	736,017,000
Collateralized Debt Obligations [Member] | Total Fair Value [Member]
Financial assets:
Fixed income securities	50,958,000	51,037,000
Collateralized Debt Obligations [Member] | Reported Value Measurement [Member]
Financial assets:
Fixed income securities	50,958,000	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	50,958,000	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	589,648,000	597,942,000
Asset-backed Securities [Member] | Reported Value Measurement [Member]
Financial assets:
Fixed income securities	589,648,000	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	517,327,000	525,402,000
Asset-backed Securities [Member] | Level 3 [Member]
Financial assets:
Fixed income securities	72,321,000	72,540,000
Short-Term [Member] | Total Fair Value [Member]
Financial assets:
Short term investments	321,119,000	557,270,000
Short-Term [Member] | Reported Value Measurement [Member]
Financial assets:
Short term investments	321,119,000	557,270,000
Short-Term [Member] | Level 1 [Member]
Financial assets:
Short term investments	269,171,000	389,299,000
Short-Term [Member] | Level 2 [Member]
Financial assets:
Short term investments	51,948,000	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	402,072,000	413,977,000
Other [Member] | Reported Value Measurement [Member]
Financial assets:
Other investments	419,896,000	431,630,000
Other [Member] | Level 1 [Member]
Financial assets:
Other investments	57,535,000	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	18,385,000	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	70,566,000
Interest Rate Swap [Member] | Total Fair Value [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial liabilities:
Derivative liabilities	2,155,456,000
Interest Rate Swap [Member] | Reported Value Measurement [Member]
Financial liabilities:
Derivative liabilities	70,566,000
Interest Rate Swap [Member] | Reported Value Measurement [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial liabilities:
Derivative liabilities	2,155,456,000
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	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	70,566,000
Interest Rate Swap [Member] | Level 2 [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial liabilities:
Derivative liabilities	2,155,456,000
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	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	1,018,000		566,000
Credit Derivatives [Member] | Reported Value Measurement [Member] | Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities	1,018,000		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	1,018,000		$ 566,000
Investments Measured at NAV as a Practical Expedient [Member]
Financial assets:
Other investments	$ 326,152,000	$ 310,441,000