Agency Mortgage-Backed Securities - Narrative (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Mortgage-Backed Securities Sold:
Mortgage-Backed securities sold, carrying value	$ 7,000,000,000	$ 3,600,000,000	$ 17,900,000,000	$ 8,500,000,000
Mortgage-Backed securities sold, realized gain	142,000,000	91,700,000	317,100,000	118,500,000
Interest-only securities [Member]
Mortgage-Backed Securities Sold:
Unrealized losses	129,900,000		129,900,000
Amortized cost	$ 604,500,000		$ 604,500,000