Credit Quality (Summary of Loans Modified in a TDR) (Detail)		12 Months Ended
		Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Financing Receivable, Modifications
Number of Loans Modified in a TDR During the Period	[1],[2]	9,256	10,978	14,472
Recorded Investment in Loans Modified in a TDR During the Period	[2]	$ 413,000,000	$ 413,000,000	$ 409,000,000
Increase (Decrease) to ALLL Upon Modification	[2]	13,000,000	32,000,000	23,000,000
Charge-offs Recognized Upon Modification	[2]	$ 7,000,000	$ 4,000,000	$ 10,000,000
Commercial Portfolio Segment | Commercial and Industrial Loans
Financing Receivable, Modifications
Number of Loans Modified in a TDR During the Period	[1],[2]	75	74	77
Recorded Investment in Loans Modified in a TDR During the Period	[2]	$ 237,000,000	$ 183,000,000	$ 146,000,000
Increase (Decrease) to ALLL Upon Modification	[2]	(5,000,000)	14,000,000	7,000,000
Charge-offs Recognized Upon Modification	[2]	$ 6,000,000	$ 0	$ 3,000,000
Commercial Portfolio Segment | Commercial Mortgage Loans, Owner-Occupied
Financing Receivable, Modifications
Number of Loans Modified in a TDR During the Period	[1],[2]	9	12	18
Recorded Investment in Loans Modified in a TDR During the Period	[2]	$ 8,000,000	$ 11,000,000	$ 16,000,000
Increase (Decrease) to ALLL Upon Modification	[2]	5,000,000	0	(2,000,000)
Charge-offs Recognized Upon Modification	[2]	$ 0	$ 0	$ 0
Commercial Portfolio Segment | Commercial Mortgage Loans, Nonowner-Occupied
Financing Receivable, Modifications
Number of Loans Modified in a TDR During the Period	[1],[2]	4	4	12
Recorded Investment in Loans Modified in a TDR During the Period	[2]	$ 0	$ 5,000,000	$ 7,000,000
Increase (Decrease) to ALLL Upon Modification	[2]	0	2,000,000	(1,000,000)
Charge-offs Recognized Upon Modification	[2]	$ 0	$ 0	$ 0
Commercial Portfolio Segment | Commercial Leases
Financing Receivable, Modifications
Number of Loans Modified in a TDR During the Period	[1],[2]	1	5
Recorded Investment in Loans Modified in a TDR During the Period	[2]	$ 4,000,000	$ 16,000,000
Increase (Decrease) to ALLL Upon Modification	[2]	0	0
Charge-offs Recognized Upon Modification	[2]	$ 0	$ 0
Residential Mortgage Loans
Financing Receivable, Modifications
Number of Loans Modified in a TDR During the Period	[1],[2]	830	924	1,089
Recorded Investment in Loans Modified in a TDR During the Period	[2]	$ 116,000,000	$ 137,000,000	$ 155,000,000
Increase (Decrease) to ALLL Upon Modification	[2]	5,000,000	8,000,000	8,000,000
Charge-offs Recognized Upon Modification	[2]	$ 0	$ 0	$ 0
Consumer Portfolio Segment | Home Equity
Financing Receivable, Modifications
Number of Loans Modified in a TDR During the Period	[1],[2]	150	219	267
Recorded Investment in Loans Modified in a TDR During the Period	[2]	$ 10,000,000	$ 15,000,000	$ 16,000,000
Increase (Decrease) to ALLL Upon Modification	[2]	0	0	(1,000,000)
Charge-offs Recognized Upon Modification	[2]	$ 0	$ 0	$ 0
Consumer Portfolio Segment | Automobile Loans
Financing Receivable, Modifications
Number of Loans Modified in a TDR During the Period	[1],[2]	102	221	440
Recorded Investment in Loans Modified in a TDR During the Period	[2]	$ 0	$ 3,000,000	$ 7,000,000
Increase (Decrease) to ALLL Upon Modification	[2]	0	0	1,000,000
Charge-offs Recognized Upon Modification	[2]	$ 0	$ 0	$ 0
Consumer Portfolio Segment | Credit Card
Financing Receivable, Modifications
Number of Loans Modified in a TDR During the Period	[1],[2]	8,085	9,519	12,569
Recorded Investment in Loans Modified in a TDR During the Period	[2]	$ 38,000,000	$ 43,000,000	$ 62,000,000
Increase (Decrease) to ALLL Upon Modification	[2]	8,000,000	8,000,000	11,000,000
Charge-offs Recognized Upon Modification	[2]	$ 1,000,000	$ 4,000,000	$ 7,000,000

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

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