Financial Guarantee Insurance Contracts - Summary of Rollforward of RMBS Subrogation, by Estimation Approach (Detail) (USD $) In Thousands, unless otherwise specified	5 Months Ended	9 Months Ended	4 Months Ended	5 Months Ended	9 Months Ended	4 Months Ended	5 Months Ended	9 Months Ended	4 Months Ended
	Sep. 30, 2013 Successor Ambac [Member]	Sep. 30, 2014 Successor Ambac [Member]	Apr. 30, 2013 Predecessor Ambac [Member]	Sep. 30, 2013 Random Samples [Member] Successor Ambac [Member]	Sep. 30, 2014 Random Samples [Member] Successor Ambac [Member]	Apr. 30, 2013 Random Samples [Member] Predecessor Ambac [Member]	Sep. 30, 2013 Adverse Samples [Member] Successor Ambac [Member]	Sep. 30, 2014 Adverse Samples [Member] Successor Ambac [Member]	Apr. 30, 2013 Adverse Samples [Member] Predecessor Ambac [Member]
Subrogation By Estimation Approach [Roll Forward]
Discounted RMBS subrogation (gross of reinsurance), beginning balance	$ 2,482,918	$ 2,206,598	$ 2,523,225	$ 1,004,252	$ 953,825	$ 1,080,408	$ 1,478,666	$ 1,252,773	$ 1,442,817
Changes recognized
Additional Transactions Reviewed Changes in RMBS Subrogation	2,426	24,565	0	2,426	24,565
Changes in Estimation Approach In Rmbs Subrogation	0	53,851	0	0	1,272,532	0	0	(1,218,681)	0
Impact of sponsor actions	0	(90,000)	(54,195)	0	(90,000)	(54,195)	0	0	0
All other changes	(123,488)	19,416	13,888	(48,910)	53,508	(21,961)	(74,578)	(34,092)	35,849
Discounted RMBS subrogation (gross of reinsurance), ending balance	$ 2,361,856	$ 2,214,430	$ 2,482,918	$ 957,768	$ 2,214,430	$ 1,004,252	$ 1,404,088	$ 0	$ 1,478,666