Segment Information - Summary of Segment Information (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Segment Reporting Information [Line Items]
Average Total Assets	$ 117,684	$ 120,860	$ 122,978
Business Banking [Member]
Segment Reporting Information [Line Items]
Average Total Assets	5,681	5,602	5,596
Commercial Banking [Member]
Segment Reporting Information [Line Items]
Average Total Assets	26,482	26,573	26,723
Commercial Real Estate [Member]
Segment Reporting Information [Line Items]
Average Total Assets	22,811	22,741	22,977
Discretionary Portfolio [Member]
Segment Reporting Information [Line Items]
Average Total Assets	34,095	37,203	38,731
Residential Mortgage Banking [Member]
Segment Reporting Information [Line Items]
Average Total Assets	2,278	2,355	2,426
Retail Banking [Member]
Segment Reporting Information [Line Items]
Average Total Assets	13,377	12,702	12,204
All Other [Member]
Segment Reporting Information [Line Items]
Average Total Assets	$ 12,960	$ 13,684	$ 14,321