Segment Information - Summary of Segment Information (Detail) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Segment Reporting Information [Line Items]
Average Total Assets	$ 148,157	$ 135,480	$ 120,585
Business Banking [Member]
Segment Reporting Information [Line Items]
Average Total Assets	8,622	8,152	6,006
Commercial Banking [Member]
Segment Reporting Information [Line Items]
Average Total Assets	30,395	30,338	29,172
Commercial Real Estate [Member]
Segment Reporting Information [Line Items]
Average Total Assets	26,097	25,792	24,849
Discretionary Portfolio [Member]
Segment Reporting Information [Line Items]
Average Total Assets	23,650	27,726	27,143
Residential Mortgage Banking [Member]
Segment Reporting Information [Line Items]
Average Total Assets	6,506	4,038	3,218
Retail Banking [Member]
Segment Reporting Information [Line Items]
Average Total Assets	17,216	16,438	16,002
All Other [Member]
Segment Reporting Information [Line Items]
Average Total Assets	$ 35,671	$ 22,996	$ 14,195