Agency Mortgage-Backed Securities - Narrative (Detail) (USD $)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014 Interest-only securities	Dec. 31, 2013 Interest-only securities	Mar. 31, 2014 Accumulated Other Comprehensive Income (Loss) Interest-only securities	Dec. 31, 2013 Accumulated Other Comprehensive Income (Loss) Interest-only securities
Mortgage-Backed Securities Sold:
Mortgage-Backed securities sold, carrying value	$ 4,300,000,000	$ 16,300,000,000
Mortgage-Backed securities sold, realized gain	129,500,000	182,800,000
Unrealized gains					57,300,000	78,100,000
Amortized cost			$ 1,100,000,000	$ 1,000,000,000