Derivative Instruments - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified			9 Months Ended		9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012 Call Option on Long-Term Debt [Member]	Sep. 30, 2013 Call Option on Long-Term Debt [Member]	Sep. 30, 2013 Credit Derivatives [Member] Contract	Dec. 31, 2012 Credit Derivatives [Member] 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"	$ 5,599	$ 5,472
Value of obligation to return cash collateral, recorded in "Other liabilities"	0	0
Number of outstanding credit derivative transactions include ratings based collateral-posting triggers					None of our outstanding credit derivative transactions	None of our outstanding credit derivative transactions
Number of credit derivative contracts					4	4
Net liability fair value of credit derivative contracts					67,164	67,219
Notional value of credit derivative contracts					281,547	291,690
Gains in change in fair value of the call options			100,710
Amount of surplus notes that may be repurchased under stand-alone derivatives				500,000
Net liability fair value of all derivative instruments linked to Ambac's own credit risk	69,513	180,113
Fair value of posted assets as collateral	$ 140,273	$ 271,251