Agency Mortgage-Backed Securities - Portfolio (Detail) (USD $)	Jun. 30, 2011	Dec. 31, 2010
Mortgage-Backed Securities Portfolio [Line Items]
Mortgage-Backed Securities, gross	$ 91,732,681,000	$ 75,011,712,000
Unamortized discount	(29,737,000)	(33,383,000)
Unamortized premium	3,017,765,000	2,338,823,000
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]
Freddie Mac [Member]
Mortgage-Backed Securities Portfolio [Line Items]
Mortgage-Backed Securities, gross	25,057,495,000	19,846,543,000
Unamortized discount	(12,488,000)	(14,651,000)
Unamortized premium	660,423,000	517,507,000
Amortized cost	25,705,430,000	20,349,399,000
Gross Unrealized Gain	693,858,000	463,471,000
Gross Unrealized Loss	(38,608,000)	(140,027,000)
Estimated fair value	26,360,680,000	20,672,843,000
Fannie Mae [Member]
Mortgage-Backed Securities Portfolio [Line Items]
Mortgage-Backed Securities, gross	65,731,429,000	54,341,140,000
Unamortized discount	(16,848,000)	(18,329,000)
Unamortized premium	2,323,888,000	1,795,116,000
Amortized cost	68,038,469,000	56,117,927,000
Gross Unrealized Gain	1,629,067,000	1,211,324,000
Gross Unrealized Loss	(267,619,000)	(438,918,000)
Estimated fair value	69,399,917,000	56,890,333,000
Ginnie Mae [Member]
Mortgage-Backed Securities Portfolio [Line Items]
Mortgage-Backed Securities, gross	943,757,000	824,029,000
Unamortized discount	(401,000)	(403,000)
Unamortized premium	33,454,000	26,200,000
Amortized cost	976,810,000	849,826,000
Gross Unrealized Gain	37,683,000	29,408,000
Gross Unrealized Loss	(1,642,000)	(2,080,000)
Estimated fair value	$ 1,012,851,000	$ 877,154,000

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