Note Allowance for loan losses (Troubled debt restructurings loan count by type of modification) (Detail) - loans	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Reduction In Interest Rate	352	502	1,206	1,511
Extension of Maturity Date	35	35	98	93
Combination Interest Rate Reduction Maturity Date Extension	93	140	320	388
Other Modifications	132	203	545	668
CRE non-owner occupied
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Reduction In Interest Rate	0	3	4	5
Extension of Maturity Date	0	0	1	1
Combination Interest Rate Reduction Maturity Date Extension	0	0	0	0
Other Modifications	0	0	0	0
CRE owner occupied
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Reduction In Interest Rate	0	9	3	38
Extension of Maturity Date	3	0	12	5
Combination Interest Rate Reduction Maturity Date Extension	0	0	0	0
Other Modifications	0	0	0	0
Commercial and industrial
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Reduction In Interest Rate	1	8	3	22
Extension of Maturity Date	15	0	36	1
Combination Interest Rate Reduction Maturity Date Extension	0	0	0	0
Other Modifications	0	0	0	0
Mortgages
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Reduction In Interest Rate	13	17	45	55
Extension of Maturity Date	14	24	35	58
Combination Interest Rate Reduction Maturity Date Extension	83	134	301	376
Other Modifications	16	43	116	133
Leasing
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Reduction In Interest Rate	0	0	0	0
Extension of Maturity Date	0	1	1	1
Combination Interest Rate Reduction Maturity Date Extension	1	0	6	0
Other Modifications	0	0	0	0
Credit cards
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Reduction In Interest Rate	140	218	425	603
Extension of Maturity Date	0	0	0	0
Combination Interest Rate Reduction Maturity Date Extension	4	1	5	1
Other Modifications	114	158	424	531
Home equity lines of credit
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Reduction In Interest Rate	0	0	0	0
Extension of Maturity Date	0	0	1	0
Combination Interest Rate Reduction Maturity Date Extension	2	0	3	2
Other Modifications	0	0	0	1
Personal
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Reduction In Interest Rate	187	241	699	761
Extension of Maturity Date	2	6	6	16
Combination Interest Rate Reduction Maturity Date Extension	1	1	1	1
Other Modifications	2	0	3	1
Auto
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Reduction In Interest Rate	0	0	0	0
Extension of Maturity Date	1	4	5	11
Combination Interest Rate Reduction Maturity Date Extension	2	4	4	8
Other Modifications	0	2	1	2
Other
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Reduction In Interest Rate	11	6	27	27
Extension of Maturity Date	0	0	1	0
Combination Interest Rate Reduction Maturity Date Extension	0	0	0	0
Other Modifications	0	0	1	0