Credit Quality (Summary of Loans Modified in a TDR) (Detail) $ in Millions	3 Months Ended
Mar. 31, 2020 USD ($) bps	Mar. 31, 2019 USD ($)
Financing Receivable, Modifications
Number of Loans Modified in a TDR During the Period	2,156	1,611
Recorded Investment in Loans Modified in a TDR During the Period	$ 120	$ 65
Increase (Decrease) to ALLL Upon Modification	13	(3)
Charge-offs Recognized Upon Modification	$ 0	$ 1
Commercial Portfolio Segment \| Commercial and Industrial Loans
Financing Receivable, Modifications
Number of Loans Modified in a TDR During the Period	30	13
Recorded Investment in Loans Modified in a TDR During the Period	$ 69	$ 34
Increase (Decrease) to ALLL Upon Modification	10	(5)
Charge-offs Recognized Upon Modification	$ 0	$ 0
Commercial Portfolio Segment \| Commercial Mortgage Loans, Owner-Occupied
Financing Receivable, Modifications
Number of Loans Modified in a TDR During the Period	11	3
Recorded Investment in Loans Modified in a TDR During the Period	$ 7	$ 4
Increase (Decrease) to ALLL Upon Modification	0	0
Charge-offs Recognized Upon Modification	$ 0	$ 0
Commercial Portfolio Segment \| Commercial Mortgage Loans, Nonowner-Occupied
Financing Receivable, Modifications
Number of Loans Modified in a TDR During the Period \| bps	3
Recorded Investment in Loans Modified in a TDR During the Period	$ 8
Increase (Decrease) to ALLL Upon Modification	0
Charge-offs Recognized Upon Modification	$ 0
Commercial Portfolio Segment \| Commercial Construction Loans
Financing Receivable, Modifications
Number of Loans Modified in a TDR During the Period \| bps	1
Recorded Investment in Loans Modified in a TDR During the Period	$ 0
Increase (Decrease) to ALLL Upon Modification	0
Charge-offs Recognized Upon Modification	$ 0
Residential Mortgage Loans
Financing Receivable, Modifications
Number of Loans Modified in a TDR During the Period	184	136
Recorded Investment in Loans Modified in a TDR During the Period	$ 24	$ 18
Increase (Decrease) to ALLL Upon Modification	0	0
Charge-offs Recognized Upon Modification	$ 0	$ 0
Consumer Portfolio Segment \| Home Equity
Financing Receivable, Modifications
Number of Loans Modified in a TDR During the Period	21	21
Recorded Investment in Loans Modified in a TDR During the Period	$ 2	$ 1
Increase (Decrease) to ALLL Upon Modification	(1)	0
Charge-offs Recognized Upon Modification	$ 0	$ 0
Consumer Portfolio Segment \| Indirect secured consumer loans
Financing Receivable, Modifications
Number of Loans Modified in a TDR During the Period	22	29
Recorded Investment in Loans Modified in a TDR During the Period	$ 0	$ 0
Increase (Decrease) to ALLL Upon Modification	0	0
Charge-offs Recognized Upon Modification	$ 0	$ 0
Consumer Portfolio Segment \| Credit Card
Financing Receivable, Modifications
Number of Loans Modified in a TDR During the Period	1,884	1,409
Recorded Investment in Loans Modified in a TDR During the Period	$ 10	$ 8
Increase (Decrease) to ALLL Upon Modification	4	2
Charge-offs Recognized Upon Modification	$ 0	$ 1

Amount of other addition (reduction) in allowance reserve for credit loss.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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