Agency Mortgage-Backed Securities - Narrative (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Mortgage-Backed Securities Sold:
Mortgage-Backed securities sold, carrying value	$ 20,600,000,000	$ 54,500,000,000	$ 30,400,000,000
Mortgage-Backed securities sold, realized gain	179,700,000	440,200,000	438,500,000
Interest-only securities
Mortgage-Backed Securities Sold:
Amortized cost	1,200,000,000	1,000,000,000
Accumulated Other Comprehensive Income (Loss) | Interest-only securities
Mortgage-Backed Securities Sold:
Unrealized gains (Losses)	$ (8,000,000)	$ 78,100,000