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=109223946&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=109226348&loc=d3e2611-110228

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

Bank Premises & Equipment (Bank Premises and Equipment - Additional Information) (Detail) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)
Bank Premises and Equipment
Bank premises impairment	$ 1	$ 41	$ 6
2018 Branch Optimization Plan
Bank Premises and Equipment
Land and improvements		21
Branches held for sale		31
2018 Branch Optimization Plan \| Lower Limit
Bank Premises and Equipment
Branches held for sale		100
2018 Branch Optimization Plan \| Upper Limit
Bank Premises and Equipment
Branches held for sale		125