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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
Dec. 31, 2012
Transactions
Dec. 31, 2011
Transactions
Schedule Of Balance Of Rmbs Subrogation Recoveries And Related Claim Liabilities By Estimation Approach [Line Items]    
Count 49 46
Gross loss reserve before subrogation recoveries $ 3,563,344 $ 3,777,581
Subrogation recoveries (2,523,225) (2,720,266)
Gross loss reserve after subrogation recoveries 1,040,119 1,057,315
Adverse samples [Member]
    
Schedule Of Balance Of Rmbs Subrogation Recoveries And Related Claim Liabilities By Estimation Approach [Line Items]    
Count 27 30
Gross loss reserve before subrogation recoveries 2,331,878 2,637,479
Subrogation recoveries (1,442,817) (1,457,472)
Gross loss reserve after subrogation recoveries 889,061 1,180,007
Random samples [Member]
    
Schedule Of Balance Of Rmbs Subrogation Recoveries And Related Claim Liabilities By Estimation Approach [Line Items]    
Count 22 16
Gross loss reserve before subrogation recoveries 1,231,466 1,140,102
Subrogation recoveries (1,080,408) (1,262,794)
Gross loss reserve after subrogation recoveries $ 151,058 $ (122,692)