Securities - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Investment Holdings [Line Items]
Trading securities		$ 374		$ 360
Securities with a fair value, pledged as collateral		11,300		$ 14,200
Available-for-sale Securities
Investment Holdings [Line Items]
Marketable Securities, Realized Loss, Other than Temporary Impairments, Amount	$ 7	$ 5	$ 24