Securities Available for Sale - Additional Information (Detail) - USD ($) $ in Millions		Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities Pledged as Collateral		$ 4,300	$ 2,000
Available-for-sale Securities	[1]	$ 31,416	$ 30,672

[1] Includes securities AFS pledged as collateral where counterparties have the right to sell or repledge the collateral of $223 million and $0 million at December 31, 2017 and December 31, 2016, respectively.