Derivative Instruments - Summarize Information by Major Category of CDS Contracts (Detail) (USD $) In Thousands, unless otherwise specified			12 Months Ended		9 Months Ended		12 Months Ended		9 Months Ended		12 Months Ended		9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Dec. 31, 2012 Predecessor Ambac [Member] Transactions	Dec. 31, 2012 Predecessor Ambac [Member] Credit Derivatives [Member]	Sep. 30, 2013 Successor Ambac [Member] Transactions	Sep. 30, 2013 Successor Ambac [Member] Credit Derivatives [Member]	Dec. 31, 2012 CLO [Member] Predecessor Ambac [Member] Transactions	Dec. 31, 2012 CLO [Member] Predecessor Ambac [Member] Credit Derivatives [Member]	Sep. 30, 2013 CLO [Member] Successor Ambac [Member] Transactions	Sep. 30, 2013 CLO [Member] Successor Ambac [Member] Credit Derivatives [Member]	Dec. 31, 2012 Other [Member] Predecessor Ambac [Member] Transactions	Dec. 31, 2012 Other [Member] Predecessor Ambac [Member] Credit Derivatives [Member]	Sep. 30, 2013 Other [Member] Successor Ambac [Member] Transactions	Sep. 30, 2013 Other [Member] Successor Ambac [Member] Credit Derivatives [Member]
Summary of information by major category of CDS contracts
Number of CDS transactions			51		26		30		8		21		18
Remaining expected weighted-average life of obligations (in years)			3 years 7 months 6 days		4 years 4 months 24 days		2 years 2 months 12 days		2 years 3 months 18 days		5 years 2 months 12 days		5 years 3 months 18 days
Gross principal notional outstanding	$ 5,207,364	$ 11,281,777	$ 11,281,777		$ 5,207,364		$ 6,155,767		$ 1,609,378		$ 5,126,010		$ 3,597,986
Net derivative liabilities at fair value				$ (213,585)		$ (202,653)		$ (34,645)		$ (10,201)		$ (178,940)		$ (192,452)