Bank Premises and Equipment - Additional Information (Details) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2021 USD ($) branch	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) branch	Sep. 30, 2020 USD ($)
Property, Plant and Equipment [Line Items]
Impairment losses on bank premises \| $	$ 3	$ 11	$ 7	$ 25
Closed In 2021
Property, Plant and Equipment [Line Items]
Number of branches held for sale \| branch	44		44

The aggregate amount of write-downs for impairments recognized during the period for long-lived assets held for abandonment, exchange or sale.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=123570139&loc=d3e3602-108585

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 45
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=123351718&loc=d3e2611-110228

Name:	us-gaap_ImpairmentOfLongLivedAssetsToBeDisposedOf
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration