Segment Information - Summary of Segment Information (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Segment Reporting Information [Line Items]
Average Total Assets	$ 150,967	$ 135,480	$ 132,430
Business Banking [Member]
Segment Reporting Information [Line Items]
Average Total Assets	8,386	8,152	7,902
Commercial Banking [Member]
Segment Reporting Information [Line Items]
Average Total Assets	29,109	30,338	30,450
Commercial Real Estate [Member]
Segment Reporting Information [Line Items]
Average Total Assets	25,913	25,792	25,594
Discretionary Portfolio [Member]
Segment Reporting Information [Line Items]
Average Total Assets	22,317	27,726	27,878
Residential Mortgage Banking [Member]
Segment Reporting Information [Line Items]
Average Total Assets	6,582	4,038	3,530
Retail Banking [Member]
Segment Reporting Information [Line Items]
Average Total Assets	17,701	16,438	16,231
All Other [Member]
Segment Reporting Information [Line Items]
Average Total Assets	$ 40,959	$ 22,996	$ 20,845