Credit Quality (Summary of Average Impaired Loans and Leases and Interest Income by Class) (Parenthetical) (Detail) $ in Millions	3 Months Ended			9 Months Ended				12 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)		Sep. 30, 2018 USD ($)		Sep. 30, 2017 USD ($)		Dec. 31, 2017
Financing Receivable, Impaired
Average Recorded Investment	$ 1,430	$ 1,660		$ 1,513		$ 1,738
Number of contracts	1,803	2,353		6,347		6,842
Commercial Portfolio Segment | Commercial Mortgage Loans, Owner-occupied
Financing Receivable, Impaired
Average Recorded Investment	$ 11	$ 50		$ 17		$ 55
Number of contracts		1	[1],[2],[3],[4]	2	[5],[6],[7],[8]	8	[1],[2],[3],[4]
Commercial Portfolio Segment | Commercial Mortgage Loans, Owner-occupied | Variable Interest Entity, Primary Beneficiary
Financing Receivable, Impaired
Number of contracts		5						5

[1] Represents number of loans post-modification and excludes loans previously modified in a TDR .

[2] Represents number of loans post-modification and excludes loans previously modified in a TDR .

[3] Excludes all loans and leases held for sale and loans acquired with deteriorated credit quality which were accounted for within a pool .

[4] Excludes all loans and leases held for sale and loans acquired with deteriorated credit quality which were accounted for within a pool .

[5] Represents number of loans post-modification and excludes loans previously modified in a TDR .

[6] Represents number of loans post-modification and excludes loans previously modified in a TDR .

[7] Excludes all loans and leases held for sale and loans acquired with deteriorated credit quality which were accounted for within a pool .

[8] Excludes all loans and leases held for sale and loans acquired with deteriorated credit quality which were accounted for within a pool .