Business Combination - Merger-Related Expenses (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Business Acquisition [Line Items]
Compensation and benefits	[1]	$ 2,590	$ 2,418	$ 2,115
Technology and communications	[1]	362	422	285
Net occupancy expense	[1]	350	332	292
Equipment expense	[1]	130	129	123
Card and processing expense	[1]	121	130	123
Marketing expense	[1]	104	162	147
Other noninterest expense	[1]	921	934	797
Total noninterest expense	[1]	4,718	4,660	$ 3,958
MB Financial, Inc.
Business Acquisition [Line Items]
Compensation and benefits		4	90
Technology and communications		6	71
Net occupancy expense		4	13
Equipment expense		0	1
Card and processing expense		0	1
Marketing expense		0	7
Other noninterest expense		2	39
Total noninterest expense		$ 16	$ 222

[1] During the first quarter of 2020, certain noninterest income and noninterest expense line items were reclassified to better align disclosures to business activities. These reclassifications were retrospectively applied to all prior periods presented. Total noninterest income and noninterest expense did not change as a result of these reclassifications.