Business Combination - Merger-Related Expenses (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Business Acquisition [Line Items]
Compensation and benefits	[1]	$ 637	$ 584	$ 1,911	$ 1,843
Technology and communications	[1]	89	100	272	319
Net occupancy expense	[1]	90	84	254	248
Equipment expense	[1]	33	33	97	96
Card and processing expense	[1]	29	33	89	98
Marketing expense	[1]	23	40	74	117
Other noninterest expense	[1]	225	245	682	681
Total noninterest expense	[1]	1,161	1,159	3,482	3,499
MB Financial, Inc.
Business Acquisition [Line Items]
Compensation and benefits		0	14	4	88
Technology and communications		0	8	6	68
Net occupancy expense		0	3	4	10
Equipment expense		0	0	0	1
Card and processing expense		0	0	0	1
Marketing expense		0	0	0	7
Other noninterest expense		$ 0	3	2	38
Total noninterest expense			$ 28	$ 16	$ 213

[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.