Securities - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Investment Holdings
Trading securities	$ 246,000,000		$ 246,000,000		$ 207,000,000
Securities with a fair value, pledged as collateral	8,900,000,000		8,900,000,000		12,600,000,000
Gross realized gains on trading securities			1,000,000	1,000,000
Marketable Securities, Realized Loss, Other than Temporary Impairments, Amount	45,000,000	23,000,000	57,000,000	39,000,000
Net Unrealized Gain(Loss) on Trading Securities	2,000,000		3,000,000	1,000,000
Available-for-sale Securities
Investment Holdings
Marketable Securities, Realized Loss, Other than Temporary Impairments, Amount	$ 45,000,000	$ 23,000,000	$ 57,000,000	$ 39,000,000