Segment Information - Summary of Segment Information (Detail) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Segment Reporting Information [Line Items]
Average Total Assets	$ 118,584	$ 116,959	$ 116,676
Business Banking [Member]
Segment Reporting Information [Line Items]
Average Total Assets	5,758	5,631	5,637
Commercial Banking [Member]
Segment Reporting Information [Line Items]
Average Total Assets	28,089	26,626	26,524
Commercial Real Estate [Member]
Segment Reporting Information [Line Items]
Average Total Assets	23,893	22,885	22,899
Discretionary Portfolio [Member]
Segment Reporting Information [Line Items]
Average Total Assets	29,355	32,123	32,659
Residential Mortgage Banking [Member]
Segment Reporting Information [Line Items]
Average Total Assets	2,444	2,161	2,123
Retail Banking [Member]
Segment Reporting Information [Line Items]
Average Total Assets	14,839	13,656	13,522
All Other [Member]
Segment Reporting Information [Line Items]
Average Total Assets	$ 14,206	$ 13,877	$ 13,312