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Financial Instruments - Notional and Fair Values of TBAs (Detail) (USD $)
In Thousands, unless otherwise specified
Sep. 30, 2013
Dec. 31, 2012
Fair Value Of To Be Announced Securities [Abstract]    
Notional Amount, Sale of TBAs $ 381,124 $ 449,065
Notional Amount, Purchase of TBAs 42,709 117,573
Notional Amount, Funding Commitments 266,273 304,390
Notional Amount, Total 308,982 421,963
Fair Value, Sale of TBAs 3,403 3,188
Fair Value, Purchase of TBAs 536 175
Fair Value, Funding Commitments      
Fair Value, Total