Background and Business Description - Additional Information (Detail)	3 Months Ended			4 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($) $ / shares shares	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Apr. 30, 2013 shares	Sep. 30, 2017 USD ($) $ / shares shares	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) Segment $ / shares shares	May 01, 2013 $ / shares shares
Background And Basis Of Presentation [Line Items]
Entity Incorporation, Date of Incorporation					Apr. 29, 1991
Investment Warrants Expiration Date				Apr. 30, 2023
Domestic Public Finance Losses					$ 434,100,000
Possible Increase in Loss Reserves Related to Puerto Rico					1,500,000,000
Ambac Assurance [Member]
Background And Basis Of Presentation [Line Items]
Net par exposure for policies allocated to the Segregated Account	$ 10,174,798,000				10,174,798,000
Minimum surplus amount	100,000,000				100,000,000
Successor [Member]
Background And Basis Of Presentation [Line Items]
Other investments	439,987,000				439,987,000		$ 450,307,000
Gain (Loss) on Sale of Derivatives	134,000	$ 226,000	$ 43,443,000		1,467,000	$ 711,000
Financial Guarantee Insurance Contracts, Accelerated Premium Revenue, Amount	$ 26,178,000	$ 18,174,000			$ 55,648,000	$ 38,231,000
Number of Reportable Segments | Segment							2
Common stock, shares authorized | shares	130,000,000				130,000,000		130,000,000
Common Stock, Par or Stated Value Per Share | $ / shares	$ 0.01				$ 0.01		$ 0.01	$ 0.01
Preferred stock, shares authorized | shares	20,000,000				20,000,000		20,000,000
Preferred Stock, Par or Stated Value Per Share | $ / shares	$ 0.01				$ 0.01		$ 0.01
Common stock, shares issued | shares	45,275,982				45,275,982		45,194,954	45,000,000
Warrants Distributed In Bankruptcy | shares				5,047,138				5,047,138
Warrants issued exercise price | $ / shares								$ 16.67
Derivative Liability	$ 90,899,000				$ 90,899,000		$ 319,286,000
Successor [Member] | Ambac UK [Member]
Background And Basis Of Presentation [Line Items]
Ballantyne Settlement with JPMIM					325,600,000
Ambac UK Benefit Through Loss Reduction					91,600,000
Previous Benefit Through Reduction of Loss and LAE					53,000,000
Total Benefit Related to Ballantyne Settlement					144,600,000
Derivative Liability Termination Payment [Domain] | Successor [Member]
Background And Basis Of Presentation [Line Items]
Payments for Derivative Instrument, Financing Activities			$ 94,407,000
Augusta [Domain] | Successor [Member]
Background And Basis Of Presentation [Line Items]
Financial Guarantee Insurance Contracts, Accelerated Premium Revenue, Amount	2,617,000
Variable Interest Entity Purchased Debt [Member]
Background And Basis Of Presentation [Line Items]
Other investments	45,000,000				45,000,000
VIRGIN ISLANDS, US
Background And Basis Of Presentation [Line Items]
HurricaneRelatedExposureNetParOutstanding	42,615,000				42,615,000
PUERTO RICO
Background And Basis Of Presentation [Line Items]
HurricaneRelatedExposureNetParOutstanding	$ 1,968,000,000				$ 1,968,000,000