Financial Guarantee Insurance Contracts - Summary of Rollforward of RMBS Subrogation, by Estimation Approach (Parenthetical) (Detail)	5 Months Ended	4 Months Ended
	Sep. 30, 2013 Successor Ambac [Member] Transactions	Apr. 30, 2013 Predecessor Ambac [Member] Adverse Samples [Member] Transactions
Summary Of Rollforward Of Subrogation By Estimation Approach [Line Items]
Number of additional transactions added to Random Sample subrogation	1
Number of additional transactions transferred from Random Sample to Adverse Sample population		2
Number of transactions removed from Adverse Sample subrogation population	2