Securities - Additional Information (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Investment Holdings [Line Items]
Trading securities	$ 200,000,000	$ 217,000,000	$ 200,000,000	$ 217,000,000	$ 177,000,000
Securities with a fair value, pledged as collateral	12,700,000,000	10,800,000,000	12,700,000,000	10,800,000,000	13,300,000,000
Gross realized gains on trading securities				1,000,000
Gross realized losses on trading securities				1,000,000
Marketable Securities, Realized Loss, Other than Temporary Impairments, Amount	17,000,000		17,000,000
Net Unrealized Gain(Loss) on Trading Securities		$ (1,000,000)	$ 1,000,000	$ (2,000,000)