Segment Information - Summary of Segment Information (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Segment Reporting Information [Line Items]
Average Total Assets	$ 132,430	$ 119,584	$ 118,584
Business Banking [Member]
Segment Reporting Information [Line Items]
Average Total Assets	7,902	5,793	5,758
Commercial Banking [Member]
Segment Reporting Information [Line Items]
Average Total Assets	30,450	28,142	28,089
Commercial Real Estate [Member]
Segment Reporting Information [Line Items]
Average Total Assets	25,594	23,921	23,893
Discretionary Portfolio [Member]
Segment Reporting Information [Line Items]
Average Total Assets	27,878	29,081	29,355
Residential Mortgage Banking [Member]
Segment Reporting Information [Line Items]
Average Total Assets	3,530	2,611	2,444
Retail Banking [Member]
Segment Reporting Information [Line Items]
Average Total Assets	16,231	15,083	14,839
All Other [Member]
Segment Reporting Information [Line Items]
Average Total Assets	$ 20,845	$ 14,953	$ 14,206