LHFI Considered Impaired (Additional Information) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Financing Receivable, Impaired [Line Items]
Financing Receivable, Modifications, Post-Modification Recorded Investment	$ 373	[1]	$ 482	[2]	$ 350	[3]
Impaired Financing Receivable, Interest Income, Cash Basis Method	4		4		7
Accrual Loans [Member]
Financing Receivable, Impaired [Line Items]
Financing Receivable, Modifications, Recorded Investment	$ 2,400		$ 2,500
Percentage Of Accruing Troubled Debt Restructurings, Current	96.00%		97.00%
Consumer Other Direct [Member]
Financing Receivable, Impaired [Line Items]
Financing Receivable, Modifications, Post-Modification Recorded Investment	$ 9	[1]	$ 50	[2],[4]	$ 1	[3]
Consumer Other Direct [Member] | Loans Modified Prior to 2016 [Member]
Financing Receivable, Impaired [Line Items]
Financing Receivable, Modifications, Post-Modification Recorded Investment			$ 41

[1] Includes loans modified under the terms of a TDR that were charged-off during the period.

[2] Includes loans modified under the terms of a TDR that were charged-off during the period.

[3] Includes loans modified under the terms of a TDR that were charged-off during the period.

[4] Includes 3,321 loans with a carrying value of $41 million that were modified prior to 2016 and reclassified as TDRs in the fourth quarter of 2016.