Agency Mortgage-Backed Securities - Portfolio Cont. (Detail) (USD $)	Jun. 30, 2011	Dec. 31, 2010
Mortgage-Backed Securities Portfolio [Line Items]
Amortized Cost	$ 94,720,709,000	$ 77,317,152,000
Gross Unrealized Gain	2,360,608,000	1,704,203,000
Gross Unrealized Loss	(307,869,000)	(581,025,000)
Estimated fair value	96,773,448,000	78,440,330,000	[1]
Adjustable Rate [Member]
Mortgage-Backed Securities Portfolio [Line Items]
Amortized Cost	9,712,428,000	10,954,627,000
Gross Unrealized Gain	345,634,000	257,822,000
Gross Unrealized Loss	(9,086,000)	(75,440,000)
Estimated fair value	10,048,976,000	11,137,009,000
Fixed Rate [Member]
Mortgage-Backed Securities Portfolio [Line Items]
Amortized Cost	85,008,281,000	66,362,525,000
Gross Unrealized Gain	2,014,974,000	1,446,381,000
Gross Unrealized Loss	(298,783,000)	(505,585,000)
Estimated fair value	$ 86,724,472,000	$ 67,303,321,000

[1] Derived from the audited consolidated financial statements at December 31, 2010.