Securities - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Investment Holdings [Line Items]
Trading securities	$ 389,000,000		$ 389,000,000		$ 343,000,000
Securities with a fair value, pledged as collateral	12,600,000,000		12,600,000,000		11,600,000,000
Gross realized gains on trading securities	3,000,000		4,000,000	1,000,000
Net Unrealized Gain(Loss) on Trading Securities	4,000,000	2,000,000	2,000,000	3,000,000
Available-for-sale Securities
Investment Holdings [Line Items]
Marketable Securities, Realized Loss, Other than Temporary Impairments, Amount	$ 7,000,000	$ 45,000,000	$ 24,000,000	$ 57,000,000