Derivative Instruments - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified			3 Months Ended	6 Months Ended		6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012 Call Option on Long-Term Debt [Member]	Jun. 30, 2012 Call Option on Long-Term Debt [Member]	Jun. 30, 2013 Call Option on Long-Term Debt [Member]	Jun. 30, 2013 Credit Derivatives [Member] Contract Derivative	Dec. 31, 2012 Credit Derivatives [Member] Contract
Derivative Instruments [Line Items]
Value of right to reclaim cash collateral and posted margin, recorded in "Other assets"	$ 0	$ 5,472
Value of obligation to return cash collateral, recorded in "Other liabilities"	1,673	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
Gross principal notional outstanding, credit derivatives not written on pay-as-you-go basis						13,346
Number of credit derivatives transactions not written on a pay-as-you-go basis						1
Net credit derivative liabilities not written on pay-as-you-go basis at fair value						8
Derivative transaction termination date for contracts not written on a pay-as-you-go basis						July 2013
Number of credit derivative contracts						4	4
Net liability fair value of credit derivative contracts						81,478	67,219
Notional value of credit derivative contracts						279,987	291,690
Gains in change in fair value of the call options			39,030	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	89,711	180,113
Fair value of posted assets as collateral	$ 164,340	$ 271,251