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Components of Carrying Value of Available-For-Sale Agency Securities (Detail) (USD $)
In Thousands, unless otherwise specified
Jun. 30, 2013
Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]    
Principal balance $ 24,662,486 $ 22,771,731
Unamortized premium 736,237 648,181
Unamortized discount (2) (4)
Gross unrealized gains 200,412 499,464
Gross unrealized losses (343,090) (121)
Carrying value/estimated fair value $ 25,256,043 $ 23,919,251