Derivative Instruments - Additional Information (Detail)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($) Contract	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) Contract	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
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		$ 0
Net liability fair value of all derivative instruments linked to Ambac's own credit risk	65,563,000		65,563,000		79,912,000
Fair value of posted assets as collateral	$ 95,291,000		$ 95,291,000		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		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"	$ 21,060,000		$ 21,060,000		20,926,000
Gains in change in fair value of the call options	186,304,000	$ 30,775,000			$ 252,687,000	$ 39,861,000
Gain (Loss) on Sale of Derivatives	$ 91,000	$ 1,134,000	$ 197,000	$ 1,333,000