Financial Guarantee Insurance Contracts - Summary of Balance of RMBS Subrogation Recoveries and Related Claim Liabilities, by Estimation Approach (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013 Successor Ambac [Member] Transactions	Apr. 30, 2013 Successor Ambac [Member]	Sep. 30, 2013 Successor Ambac [Member] Adverse Samples [Member] Transactions	Sep. 30, 2013 Successor Ambac [Member] Random Samples [Member] Transactions	Apr. 30, 2013 Predecessor Ambac [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Transactions	Dec. 31, 2011 Predecessor Ambac [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Adverse Samples [Member] Transactions	Dec. 31, 2012 Predecessor Ambac [Member] Random Samples [Member] Transactions
Schedule Of Balance Of Rmbs Subrogation Recoveries And Related Claim Liabilities By Estimation Approach [Line Items]
Count	48		27	21		49		27	22
Gross loss reserve before subrogation recoveries	$ 3,331,107		$ 2,235,026	$ 1,096,081		$ 3,563,344		$ 2,331,878	$ 1,231,466
Subrogation recoveries	(2,361,856)	(2,482,918)	(1,404,088)	(957,768)	(2,482,918)	(2,523,225)	(2,720,266)	(1,442,817)	(1,080,408)
Gross loss reserve after subrogation recoveries	$ 969,251		$ 830,938	$ 138,313		$ 1,040,119		$ 889,061	$ 151,058