Securities - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Investment Holdings [Line Items]
Trading securities	$ 205,000,000	$ 189,000,000	$ 205,000,000	$ 189,000,000	$ 177,000,000
Securities with a fair value, pledged as collateral	12,200,000,000	11,600,000,000	12,200,000,000	11,600,000,000	13,300,000,000
Gross realized gains on trading securities	1,000,000		1,000,000	1,000,000
Gross realized losses on trading securities				(1,000,000)
Marketable Securities, Realized Loss, Other than Temporary Impairments, Amount	23,000,000	9,000,000	39,000,000	9,000,000
Net Unrealized Gain(Loss) on Trading Securities		$ (8,000,000)	$ 1,000,000	$ (8,000,000)