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