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Mortgage-Backed Securities - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
3 Months Ended 9 Months Ended 9 Months Ended
Sep. 30, 2013
Sep. 30, 2013
Sep. 30, 2013
Other Forward Purchase Commitments
Dec. 31, 2012
Other Forward Purchase Commitments
Sep. 30, 2013
Minimum
Sep. 30, 2013
Maximum
Investments Classified by Contractual Maturity Date [Line Items]            
Contractual maturity of Company's agency securities, in years         15 years 30 years
Impairment of mortgage-backed securities $ 8,102 $ 8,102        
Derivatives asset     $ 5,023 $ 3,305