Securities - Additional Information (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Investment Holdings
Trading securities	$ 361,000,000		$ 361,000,000		$ 343,000,000
Securities with a fair value, pledged as collateral	13,600,000,000		13,600,000,000		11,600,000,000
Gross realized gains on trading securities			1,000,000
Marketable Securities, Realized Loss, Other than Temporary Impairments, Amount		12,000,000	17,000,000	12,000,000
Net Unrealized Gain(Loss) on Trading Securities	2,000,000		1,000,000	2,000,000
Available-for-sale Securities
Investment Holdings
Marketable Securities, Realized Loss, Other than Temporary Impairments, Amount		$ 12,000,000	$ 17,000,000	$ 12,000,000