Business Combination - Merger-Related Expenses (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Business Acquisition [Line Items]
Compensation and benefits	[1]	$ 627	$ 641	$ 1,274	$ 1,251
Technology and communications	[1]	90	136	183	219
Net occupancy expense	[1]	82	88	164	164
Equipment expense	[1]	32	33	64	63
Card and processing expense	[1]	29	34	60	64
Marketing expense	[1]	20	41	51	77
Other noninterest expense	[1]	208	232	457	446
Total noninterest expense	[1]	1,121	1,243	2,321	2,341
MB Financial, Inc.
Business Acquisition [Line Items]
Compensation and benefits		2	41	4	75
Technology and communications		4	49	6	60
Net occupancy expense		2	6	4	6
Equipment expense		0	1	0	1
Card and processing expense		0	1	0	1
Marketing expense		0	3	0	6
Other noninterest expense		1	8	2	36
Total noninterest expense		$ 9	$ 109	$ 16	$ 185

[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.