Securities - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Investment Holdings [Line Items]
Trading securities	$ 370		$ 370		$ 360
Securities with a fair value, pledged as collateral	12,500		12,500		$ 14,200
Gross realized gains on trading securities			1	$ 1
Net Unrealized Gain(Loss) on Trading Securities	(1)	$ 2		1
Available-for-sale Securities
Investment Holdings [Line Items]
Marketable Securities, Realized Loss, Other than Temporary Impairments, Amount	$ 4		$ 5	$ 17