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Mortgage-Backed Securities - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Other Forward Purchase Commitments
Dec. 31, 2011
Other Forward Purchase Commitments
Dec. 31, 2012
Minimum
Dec. 31, 2012
Maximum
Investments Classified by Contractual Maturity Date [Line Items]        
Contractual maturity of Company's agency securities, in years     15 years 30 years
Derivatives $ 3,305 $ 4,015