Business Combination - Merger-Related Expenses (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Business Acquisition [Line Items]
Salaries, wages and incentives	$ 544	$ 471	$ 1,023	$ 918
Employee benefits	97	78	228	188
Technology and communications	136	67	219	135
Net occupancy expense	88	74	164	149
Card and processing expense	34	30	64	60
Equipment expense	33	30	63	61
Other noninterest expense	311	251	580	500
Total noninterest expense	1,243	$ 1,001	2,341	$ 2,011
MB Financial, Inc.
Business Acquisition [Line Items]
Salaries, wages and incentives	41		72
Employee benefits	0		3
Technology and communications	49		60
Net occupancy expense	6		6
Card and processing expense	1		1
Equipment expense	1		1
Other noninterest expense	11		42
Total noninterest expense	$ 109		$ 185