Segment Information - Summary of Segment Information (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Segment Reporting Information [Line Items]
Average Total Assets	$ 116,676	$ 120,860	$ 121,073
Business Banking [Member]
Segment Reporting Information [Line Items]
Average Total Assets	5,637	5,602	5,596
Commercial Banking [Member]
Segment Reporting Information [Line Items]
Average Total Assets	26,524	26,573	26,699
Commercial Real Estate [Member]
Segment Reporting Information [Line Items]
Average Total Assets	22,899	22,741	22,800
Discretionary Portfolio [Member]
Segment Reporting Information [Line Items]
Average Total Assets	32,659	37,203	37,807
Residential Mortgage Banking [Member]
Segment Reporting Information [Line Items]
Average Total Assets	2,123	2,355	2,335
Retail Banking [Member]
Segment Reporting Information [Line Items]
Average Total Assets	13,522	12,702	12,519
All Other [Member]
Segment Reporting Information [Line Items]
Average Total Assets	$ 13,312	$ 13,684	$ 13,317