Securities Available for Sale - Additional Information (Detail) - USD ($) $ in Millions		3 Months Ended	6 Months Ended
		Jun. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Gains on sale of OTTI securities		$ 6	$ 6
Available-for-sale Securities Pledged as Collateral		3,300	3,300	$ 4,300
Available-for-sale Securities	[1],[2]	$ 30,942	$ 30,942	$ 30,947	[3]

[1] Beginning January 1, 2018, the Company reclassified equity securities previously presented in Securities available for sale to Other assets. Reclassifications have been made to previously reported amounts for comparability.

[2] Includes securities AFS pledged as collateral where counterparties have the right to sell or repledge the collateral of $184 million and $223 million at June 30, 2018 and December 31, 2017, respectively.

[3] Beginning January 1, 2018, the Company reclassified equity securities previously presented in Securities available for sale to Other assets on the Consolidated Balance Sheets. Reclassifications have been made to previously reported amounts for comparability. See Note 9, "Other Assets," for additional information.