Derivative Instruments - Additional Information (Detail) - Derivative Contract [Domain]	Jun. 30, 2015 USD ($) Contract	Dec. 31, 2014 USD ($) Contract
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"	$ 137,728,000	$ 158,240,000
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	81,562,000	92,869,000
Fair value of posted assets as collateral	$ 133,756,000	$ 118,844,000
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	4	4
Net liability fair value of credit derivative contracts	$ 52,228,000	$ 60,729,000
Notional value of credit derivative contracts	$ 266,451,000	$ 270,747,000