Business Combination - Merger-Related Expenses (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Business Acquisition [Line Items]
Salaries, wages and incentives	$ 497	$ 421	$ 1,527	$ 1,339
Employee benefits	87	82	316	270
Technology and communications	100	71	319	206
Net occupancy expense	84	70	248	219
Card and processing expense	33	31	98	91
Equipment expense	33	31	96	92
Other noninterest expense	325	266	895	767
Total noninterest expense	1,159	972	3,499	2,984
MB Financial, Inc.
Business Acquisition [Line Items]
Salaries, wages and incentives	14	0	85	0
Employee benefits	0	0	3	0
Technology and communications	8	0	68	0
Net occupancy expense	3	0	10	0
Card and processing expense	0	0	1	0
Equipment expense	0	0	1	0
Other noninterest expense	3	1	45	3
Total noninterest expense	$ 28	$ 1	$ 213	$ 3