Bank Premises and Equipment - Additional Information (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($) branch	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) branch	Jun. 30, 2019 USD ($)	Dec. 31, 2020 branch	Sep. 30, 2019 branch	Mar. 22, 2019 branch	Jun. 30, 2018 branch
Property, Plant and Equipment [Line Items]
Impairment losses on bank premises | $	$ 12	$ 2	$ 14	$ 22
Impairment charges resulting from acquisition | $				$ 14
Forecast
Property, Plant and Equipment [Line Items]
Number of branches held for sale					4
MB Financial, Inc. | Chicago market
Property, Plant and Equipment [Line Items]
Number of branches held for sale						45
2018 Branch Optimization Plan
Property, Plant and Equipment [Line Items]
Number of branches held for sale	126		126
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 2019
Property, Plant and Equipment [Line Items]
Number of branches held for sale	97		97
To be closed | MB Financial, Inc. | Chicago market
Property, Plant and Equipment [Line Items]
Number of branches held for sale							46
Other property | MB Financial, Inc. | Chicago market
Property, Plant and Equipment [Line Items]
Number of branches held for sale	7		7
Number of other locations held for sale	11		11
Other locations at fair value | $	$ 15		$ 15