Agency Mortgage-Backed Securities - Narrative (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Mortgage Backed Securities Sold [Line Items]
Mortgage-Backed securities sold, carrying value	$ 1,600,000,000	$ 1,400,000,000	$ 4,600,000,000	$ 3,000,000,000
Mortgage-Backed securities sold, realized gain	5,900,000	37,800,000	26,800,000	84,800,000
Interest-only strips, unrealized gains	2,360,608,000		2,360,608,000		1,704,203,000
Amortized cost, interest-only strips	94,720,709,000		94,720,709,000		77,317,152,000
Interest-only strip [Member]
Mortgage Backed Securities Sold [Line Items]
Interest-only strips, unrealized gains	276,000		276,000
Amortized cost, interest-only strips	$ 46,500,000		$ 46,500,000