Note Allowance for loan losses (Quantitative information by loan class for loans modified as T D R) (Detail) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) loans	Sep. 30, 2016 USD ($) loans	Sep. 30, 2017 USD ($) loans	Sep. 30, 2016 USD ($) loans
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	612	880	2,169	2,660
Pre-Modification Recorded Investment	$ 25,130	$ 34,706	$ 90,432	$ 116,180
Post Modification Recorded Investment	24,322	38,375	89,229	118,553
Increase (decrease) in the allowance for loan losses as a result of modification	$ 2,336	$ 4,134	$ 8,504	$ 15,656
CRE non-owner occupied
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	0	3	5	6
Pre-Modification Recorded Investment	$ 0	$ 469	$ 2,069	$ 6,989
Post Modification Recorded Investment	0	3,085	1,901	9,589
Increase (decrease) in the allowance for loan losses as a result of modification	$ 0	$ 860	$ 145	$ 5,029
CRE owner occupied
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	3	9	15	43
Pre-Modification Recorded Investment	$ 272	$ 773	$ 2,975	$ 11,623
Post Modification Recorded Investment	269	1,874	2,951	11,648
Increase (decrease) in the allowance for loan losses as a result of modification	$ 29	$ 136	$ 172	$ 473
Commercial and industrial
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	16	8	39	23
Pre-Modification Recorded Investment	$ 1,022	$ 246	$ 1,850	$ 3,832
Post Modification Recorded Investment	1,044	301	3,967	3,884
Increase (decrease) in the allowance for loan losses as a result of modification	$ 111	$ 21	$ 579	$ 1
Mortgages
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	126	218	497	622
Pre-Modification Recorded Investment	$ 17,692	$ 25,255	$ 58,777	$ 69,591
Post Modification Recorded Investment	16,633	24,681	54,965	67,702
Increase (decrease) in the allowance for loan losses as a result of modification	$ 1,103	$ 1,780	$ 3,343	$ 5,407
Leasing
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	1	1	7	1
Pre-Modification Recorded Investment	$ 27	$ 15	$ 263	$ 15
Post Modification Recorded Investment	27	15	262	15
Increase (decrease) in the allowance for loan losses as a result of modification	$ 8	$ 3	$ 74	$ 3
Credit cards
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	258	377	854	1,135
Pre-Modification Recorded Investment	$ 2,881	$ 3,321	$ 7,785	$ 10,352
Post Modification Recorded Investment	3,114	3,715	8,514	11,768
Increase (decrease) in the allowance for loan losses as a result of modification	$ 375	$ 450	$ 1,019	$ 1,677
Home equity lines of credit
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	2	0	4	3
Pre-Modification Recorded Investment	$ 203	$ 0	$ 689	$ 355
Post Modification Recorded Investment	203	0	686	398
Increase (decrease) in the allowance for loan losses as a result of modification	$ 23	$ 0	$ 36	$ 216
Personal
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	192	248	709	779
Pre-Modification Recorded Investment	$ 2,945	$ 4,481	$ 11,979	$ 13,089
Post Modification Recorded Investment	2,944	4,547	11,982	13,195
Increase (decrease) in the allowance for loan losses as a result of modification	$ 673	$ 853	$ 2,704	$ 2,784
Auto
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	3	10	10	21
Pre-Modification Recorded Investment	$ 42	$ 123	$ 2,043	$ 256
Post Modification Recorded Investment	42	134	1,999	274
Increase (decrease) in the allowance for loan losses as a result of modification	$ 8	$ 27	$ 362	$ 52
Other
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	11	6	29	27
Pre-Modification Recorded Investment	$ 46	$ 23	$ 2,002	$ 78
Post Modification Recorded Investment	46	23	2,002	80
Increase (decrease) in the allowance for loan losses as a result of modification	$ 6	$ 4	$ 70	$ 14