Financial Guarantee Insurance Contracts - Summary of Rollforward of RMBS Subrogation, by Estimation Approach (Detail) (USD $) In Thousands, unless otherwise specified	5 Months Ended	4 Months Ended	12 Months Ended	5 Months Ended	4 Months Ended	12 Months Ended	5 Months Ended	4 Months Ended	12 Months Ended
	Sep. 30, 2013 Successor Ambac [Member] Transactions	Apr. 30, 2013 Predecessor Ambac [Member] Transactions	Dec. 31, 2012 Predecessor Ambac [Member] Transactions	Sep. 30, 2013 Random Samples [Member] Successor Ambac [Member] Transactions	Apr. 30, 2013 Random Samples [Member] Predecessor Ambac [Member] Transactions	Dec. 31, 2012 Random Samples [Member] Predecessor Ambac [Member] Transactions	Sep. 30, 2013 Adverse Samples [Member] Successor Ambac [Member] Transactions	Apr. 30, 2013 Adverse Samples [Member] Predecessor Ambac [Member] Transactions	Dec. 31, 2012 Adverse Samples [Member] Predecessor Ambac [Member] Transactions
Summary Of Rollforward Of Subrogation By Estimation Approach [Line Items]
Discounted RMBS subrogation (gross of reinsurance), beginning balance	$ 2,482,918	$ 2,523,225	$ 2,720,266	$ 1,004,252	$ 1,080,408	$ 1,262,794	$ 1,478,666	$ 1,442,817	$ 1,457,472
Changes recognized
Additional transactions reviewed	2,426		18,981	2,426		54,467			(35,486)
Additional adverse sample loans reviewed
Impact of sponsor actions		(54,195)	35		(54,195)				35
All other changes	(123,488)	13,888	(216,057)	(48,910)	(21,961)	(236,853)	(74,578)	35,849	20,796
Discounted RMBS subrogation (gross of reinsurance), ending balance	$ 2,361,856	$ 2,482,918	$ 2,523,225	$ 957,768	$ 1,004,252	$ 1,080,408	$ 1,404,088	$ 1,478,666	$ 1,442,817
Discounted RMBS subrogation (gross of reinsurance), beginning balance		49		20	22	16	29	27	30
Changes recognized: Additional transactions reviewed				1		6			(1)
Additional adverse sample loans reviewed
Impact of sponsor actions
All other changes					(2)		(2)	2	(2)
Discounted RMBS subrogation (gross of reinsurance), ending balance	48		49	21	20	22	27	29	27