Derivative Instruments - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2018 USD ($) Contract	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)
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
Net liability fair value of all derivative instruments linked to Ambac's own credit risk	69,545,000		79,912,000
Fair value of posted assets as collateral	$ 98,280,000		111,391,000
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	0
Successor [Member]
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Value of right to reclaim cash collateral and posted margin, recorded in "Other assets"			$ 20,926,000	$ 22,820,000
Gains in change in fair value of the call options	$ 66,383,000	$ 9,086,000
Gain (Loss) on Sale of Derivatives	$ 106,000	$ 199,000