Securities Available for Sale - Additional Information (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Gross Realized Gains		$ 0	$ 0	$ 1	$ 4
Available-for-sale Securities Pledged as Collateral		3,300		3,300		$ 2,000
Available-for-sale Securities	[1]	$ 31,444		$ 31,444		$ 30,672

[1] Includes securities AFS pledged as collateral where counterparties have the right to sell or repledge the collateral of $280 million and $0 million at September 30, 2017 and December 31, 2016, respectively.