Business Combination - Merger-Related Expenses (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Salaries, wages and incentives	$ 2,001	$ 1,783	$ 1,633
Employee benefits	417	332	356
Technology and communications	422	285	245
Net occupancy expense	332	292	295
Card and processing expense	130	123	129
Equipment expense	129	123	117
Other noninterest expense	1,229	1,020	1,007
Total noninterest expense	4,660	3,958	$ 3,782
MB Financial, Inc.
Business Acquisition [Line Items]
Salaries, wages and incentives	87	1
Employee benefits	3	0
Technology and communications	71	6
Net occupancy expense	13	0
Card and processing expense	1	1
Equipment expense	1	0
Other noninterest expense	46	23
Total noninterest expense	$ 222	$ 31