Bank Premises and Equipment - Additional Information (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) branch	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2019 branch	Mar. 22, 2019 branch	Jun. 30, 2018 branch
Property, Plant and Equipment [Line Items]
Depreciation and amortization expense | $	$ 256	$ 255	$ 238
Impairment losses on bank premises | $	$ 30	28	$ 45
Impairment charges resulting from acquisition | $		$ 14
2018 Branch Optimization Plan | Minimum
Property, Plant and Equipment [Line Items]
Number of branches held for sale						100
2018 Branch Optimization Plan | Maximum
Property, Plant and Equipment [Line Items]
Number of branches held for sale						125
Closed in 2020
Property, Plant and Equipment [Line Items]
Number of branches held for sale	102
Closed in 2020 | MB Financial, Inc. | Chicago market
Property, Plant and Equipment [Line Items]
Number of branches held for sale	8
To be closed | MB Financial, Inc. | Chicago market
Property, Plant and Equipment [Line Items]
Number of branches held for sale					46
Closed in 2019 | MB Financial, Inc. | Chicago market
Property, Plant and Equipment [Line Items]
Number of branches held for sale				45
Other property | MB Financial, Inc. | Chicago market
Property, Plant and Equipment [Line Items]
Number of other locations held for sale	11
Other locations at fair value | $	$ 15
To be closed in First Quarter
Property, Plant and Equipment [Line Items]
Number of branches held for sale	36
To be closed in Second Quarter
Property, Plant and Equipment [Line Items]
Number of branches held for sale	7