Securities - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Investment Holdings [Line Items]
Trading securities	$ 207,000,000	$ 177,000,000
Securities with a fair value, pledged as collateral	12,600,000,000	13,300,000,000
Gross realized gains on trading securities	2,000,000	1,000,000	1,000,000
Gross realized losses on trading securities		(7,000,000)	(1,000,000)
Marketable Securities, Realized Loss, Other than Temporary Impairments, Amount	58,000,000	19,000,000	3,000,000
Net Unrealized Gain(Loss) on Trading Securities	$ 1,000,000	$ 5,000,000