Rollforward of Credit Losses Recognized in Earnings Related to Securities (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	[3]
Debt Securities, Available-for-sale [Line Items]
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Reductions, Securities Sold				$ (23)
Available-for-sale Securities, Gross Realized Gains		$ 0	$ 1	7	$ 2
Available-for-sale Securities	[1],[2]	30,984		30,984		$ 30,947
Ending balance		0	22	0	22
Available-for-sale Securities, Gross Realized Losses		0	$ 1	6	$ 1
Gains on sale of OTTI securities		$ 6		$ 6

[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 $164 million and $223 million at September 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.