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, 2018 USD ($) loans	Sep. 30, 2017 USD ($) loans	Sep. 30, 2018 USD ($) loans	Sep. 30, 2017 USD ($) loans
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	880	612	2,496	2,169
Pre-Modification Recorded Investment	$ 38,627	$ 25,130	$ 182,496	$ 90,432
Post Modification Recorded Investment	37,046	24,322	176,711	89,229
Increase (decrease) in the allowance for loan losses as a result of modification	$ 4,163	$ 2,336	$ 31,479	$ 8,504
Commercial multi-family
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	1		2
Pre-Modification Recorded Investment	$ 810		$ 1,377
Post Modification Recorded Investment	808		1,375
Increase (decrease) in the allowance for loan losses as a result of modification	$ 63		$ 106
CRE non-owner occupied
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	4		17	5
Pre-Modification Recorded Investment	$ 1,523		$ 28,969	$ 2,069
Post Modification Recorded Investment	1,521		28,908	1,901
Increase (decrease) in the allowance for loan losses as a result of modification	$ 100		$ 6,854	$ 145
CRE owner occupied
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	13	3	58	15
Pre-Modification Recorded Investment	$ 7,578	$ 272	$ 27,648	$ 2,975
Post Modification Recorded Investment	7,525	269	26,433	2,951
Increase (decrease) in the allowance for loan losses as a result of modification	$ 160	$ 29	$ 1,143	$ 172
Commercial and industrial
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	27	16	81	39
Pre-Modification Recorded Investment	$ 2,411	$ 1,022	$ 49,633	$ 1,850
Post Modification Recorded Investment	2,388	1,044	48,882	3,967
Increase (decrease) in the allowance for loan losses as a result of modification	$ 139	$ 111	$ 13,963	$ 579
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	116	126	319	497
Pre-Modification Recorded Investment	$ 15,143	$ 17,692	$ 40,741	$ 58,777
Post Modification Recorded Investment	13,507	16,633	36,442	54,965
Increase (decrease) in the allowance for loan losses as a result of modification	$ 640	$ 1,103	$ 1,874	$ 3,343
Leasing
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	3	1	4	7
Pre-Modification Recorded Investment	$ 75	$ 27	$ 98	$ 263
Post Modification Recorded Investment	73	27	96	262
Increase (decrease) in the allowance for loan losses as a result of modification	$ 23	$ 8	$ 30	$ 74
Credit cards
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	187	258	811	854
Pre-Modification Recorded Investment	$ 1,693	$ 2,881	$ 8,097	$ 7,785
Post Modification Recorded Investment	1,838	3,114	8,642	8,514
Increase (decrease) in the allowance for loan losses as a result of modification	$ 234	$ 375	$ 1,086	$ 1,019
Home equity lines of credit
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	10	2	29	4
Pre-Modification Recorded Investment	$ 913	$ 203	$ 2,638	$ 689
Post Modification Recorded Investment	906	203	2,579	686
Increase (decrease) in the allowance for loan losses as a result of modification	$ 66	$ 23	$ 440	$ 36
Personal
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	512	192	1,143	709
Pre-Modification Recorded Investment	$ 8,026	$ 2,945	$ 18,351	$ 11,979
Post Modification Recorded Investment	8,025	2,944	18,346	11,982
Increase (decrease) in the allowance for loan losses as a result of modification	$ 2,660	$ 673	$ 5,390	$ 2,704
Auto Loan
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	5	3	8	10
Pre-Modification Recorded Investment	$ 63	$ 42	$ 139	$ 2,043
Post Modification Recorded Investment	63	42	122	1,999
Increase (decrease) in the allowance for loan losses as a result of modification	$ 11	$ 8	$ 21	$ 362
Other
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	2	11	23	29
Pre-Modification Recorded Investment	$ 392	$ 46	$ 595	$ 2,002
Post Modification Recorded Investment	392	46	593	2,002
Increase (decrease) in the allowance for loan losses as a result of modification	$ 67	$ 6	$ 98	$ 70