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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
Mar. 31, 2013
Transactions
Dec. 31, 2012
Transactions
Schedule Of Balance Of Rmbs Subrogation Recoveries And Related Claim Liabilities By Estimation Approach [Line Items]    
Count 49 49
Gross loss reserve before subrogation recoveries $ 3,439,641 $ 3,563,344
Subrogation recoveries (2,487,369) (2,523,225)
Gross loss reserve after subrogation recoveries 952,272 1,040,119
Adverse samples [Member]
    
Schedule Of Balance Of Rmbs Subrogation Recoveries And Related Claim Liabilities By Estimation Approach [Line Items]    
Count 29 27
Gross loss reserve before subrogation recoveries 2,424,630 2,331,878
Subrogation recoveries (1,483,726) (1,442,817)
Gross loss reserve after subrogation recoveries 940,904 889,061
Random samples [Member]
    
Schedule Of Balance Of Rmbs Subrogation Recoveries And Related Claim Liabilities By Estimation Approach [Line Items]    
Count 20 22
Gross loss reserve before subrogation recoveries 1,015,011 1,231,466
Subrogation recoveries (1,003,643) (1,080,408)
Gross loss reserve after subrogation recoveries $ 11,368 $ 151,058