Note Allowance for loan losses (Quantitative information by loan class for loans modified as T D R) (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) loans	Mar. 31, 2017 USD ($) loans
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	592	769
Pre-Modification Recorded Investment	$ 60,666	$ 29,900
Post Modification Recorded Investment	58,441	30,642
Increase (decrease) in the allowance for loan losses as a result of modification	$ 9,539	$ 2,870
CRE non-owner occupied
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	7	1
Pre-Modification Recorded Investment	$ 22,986	$ 141
Post Modification Recorded Investment	22,923	139
Increase (decrease) in the allowance for loan losses as a result of modification	$ 6,800	$ (11)
CRE owner occupied
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	19	3
Pre-Modification Recorded Investment	$ 4,974	$ 1,157
Post Modification Recorded Investment	4,269	1,147
Increase (decrease) in the allowance for loan losses as a result of modification	$ 138	$ 56
Commercial and industrial
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	22	8
Pre-Modification Recorded Investment	$ 11,069	$ 319
Post Modification Recorded Investment	10,523	2,388
Increase (decrease) in the allowance for loan losses as a result of modification	$ (110)	$ 419
Construction
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	1
Pre-Modification Recorded Investment	$ 4,210
Post Modification Recorded Investment	4,293
Increase (decrease) in the allowance for loan losses as a result of modification	$ 474
Mortgages
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	82	192
Pre-Modification Recorded Investment	$ 10,273	$ 21,068
Post Modification Recorded Investment	8,919	19,513
Increase (decrease) in the allowance for loan losses as a result of modification	$ 457	$ 1,014
Leasing
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	0	3
Pre-Modification Recorded Investment	$ 0	$ 114
Post Modification Recorded Investment	0	115
Increase (decrease) in the allowance for loan losses as a result of modification	$ 0	$ 32
Credit cards
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	281	285
Pre-Modification Recorded Investment	$ 2,926	$ 2,402
Post Modification Recorded Investment	3,301	2,643
Increase (decrease) in the allowance for loan losses as a result of modification	$ 454	$ 312
Home equity lines of credit
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	9	0
Pre-Modification Recorded Investment	$ 865	$ 0
Post Modification Recorded Investment	856	0
Increase (decrease) in the allowance for loan losses as a result of modification	$ 267	$ 0
Personal
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	162	267
Pre-Modification Recorded Investment	$ 3,072	$ 4,598
Post Modification Recorded Investment	3,070	4,595
Increase (decrease) in the allowance for loan losses as a result of modification	$ 1,010	$ 1,033
Auto
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	1	2
Pre-Modification Recorded Investment	$ 134	$ 36
Post Modification Recorded Investment	132	37
Increase (decrease) in the allowance for loan losses as a result of modification	$ 23	$ 6
Other
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	8	8
Pre-Modification Recorded Investment	$ 157	$ 65
Post Modification Recorded Investment	155	65
Increase (decrease) in the allowance for loan losses as a result of modification	$ 26	$ 9