Derivative Instruments - Additional Information (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) Contract	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) Contract	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) Contract
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Value of obligation to return cash collateral, recorded in "Other liabilities"	$ 0		$ 0		$ 0
Net liability fair value of all derivative instruments linked to Ambac's own credit risk	81,762,000		81,762,000		82,944,000
Fair value of posted assets as collateral	$ 115,037,000		$ 115,037,000		$ 128,754,000
Credit Derivatives [Member]
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Number of credit derivative contracts | Contract	2		2		2
Net liability fair value of credit derivative contracts	$ 6,180,000		$ 6,180,000		$ 6,123,000
Notional value of credit derivative contracts	73,715,000		73,715,000		67,783,000
Successor [Member]
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Collateral Variation Margin					71,023,000
Value of right to reclaim cash collateral and posted margin, recorded in "Other assets"	24,561,000		24,561,000		$ 137,701,000
Gains in change in fair value of the call options	$ (28,482,000)	$ (188,012,000)	$ 11,379,000	$ (423,421,000)