Bank Premises and Equipment - Additional Information (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) branch	Sep. 30, 2019 USD ($) branch	Sep. 30, 2020 USD ($) branch	Sep. 30, 2019 USD ($) branch	Mar. 22, 2019 branch	Jun. 30, 2018 branch
Property, Plant and Equipment [Line Items]
Impairment losses on bank premises | $	$ 11	$ 5	$ 25	$ 27
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	100		100
Closed in 2020 | MB Financial, Inc. | Chicago market
Property, Plant and Equipment [Line Items]
Number of branches held for sale	7		7
To be closed
Property, Plant and Equipment [Line Items]
Number of branches held for sale	37		37
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		45
Other property | MB Financial, Inc. | Chicago market
Property, Plant and Equipment [Line Items]
Number of other locations held for sale	11		11
Other locations at fair value | $	$ 15		$ 15