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Agency Mortgage-Backed Securities - Weighted Average Life (Detail) (USD $)
In Thousands
Jun. 30, 2011
Dec. 31, 2010
Fair Value of Mortgage-Backed Securities according to their estimated weighted-average life:    
Less than one year $ 780,883 $ 915,398
Greater than one year and less than five years 71,716,641 59,732,123
Greater than or equal to five years 24,275,924 17,792,809
Estimated fair value 96,773,448 78,440,330 [1]
Amortized Cost of Mortgage-Backed Securities according to their estimated weighted-average life:    
Less than one year 771,339 901,824
Greater than one year and less than five years 69,613,658 58,321,570
Greater than or equal to five years 24,335,712 18,093,758
Amortized cost $ 94,720,709 $ 77,317,152
[1] Derived from the audited consolidated financial statements at December 31, 2010.