Agency Mortgage-Backed Securities - Narrative (Detail) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Mortgage-Backed Securities Sold:
Mortgage-Backed securities sold, carrying value	$ 54,500,000,000	$ 30,400,000,000	$ 18,700,000,000
Mortgage-Backed securities sold, realized gain	440,200,000	438,500,000	199,200,000
Interest-only securities
Mortgage-Backed Securities Sold:
Amortized cost	1,000,000,000	797,100,000
Accumulated Other Comprehensive Income (Loss) | Interest-only securities
Mortgage-Backed Securities Sold:
Unrealized gains	$ 78,100,000	$ (141,100,000)