Agency Mortgage-Backed Securities - Narrative (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Mortgage-Backed Securities Sold:
Mortgage-Backed securities sold, carrying value	$ 30,400,000,000	$ 18,700,000,000	$ 7,800,000,000
Mortgage-Backed securities sold, realized gain	438,500,000	199,200,000	171,600,000
Interest-only securities [Member]
Mortgage-Backed Securities Sold:
Unrealized losses	141,100,000	123,100,000
Amortized cost	$ 797,100,000	$ 405,100,000