Note Allowance for loan losses (Quantitative information by loan class for loans modified as T D R) (Detail) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) loans	Jun. 30, 2017 USD ($) loans	Jun. 30, 2018 USD ($) loans	Jun. 30, 2017 USD ($) loans
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	1,025	788	1,617	1,557
Pre-Modification Recorded Investment	$ 83,321	$ 35,402	$ 143,987	$ 65,302
Post Modification Recorded Investment	81,356	34,265	139,797	64,907
Increase (decrease) in the allowance for loan losses as a result of modification	$ 17,800	$ 3,298	$ 27,339	$ 6,168
Commercial multi-family
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	1		1
Pre-Modification Recorded Investment	$ 567		$ 567
Post Modification Recorded Investment	567		567
Increase (decrease) in the allowance for loan losses as a result of modification	$ 43		$ 43
CRE non-owner occupied
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	6	4	13	5
Pre-Modification Recorded Investment	$ 4,460	$ 1,928	$ 27,446	$ 2,069
Post Modification Recorded Investment	4,464	1,762	27,387	1,901
Increase (decrease) in the allowance for loan losses as a result of modification	$ (46)	$ 156	$ 6,754	$ 145
CRE owner occupied
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	26	9	45	12
Pre-Modification Recorded Investment	$ 15,096	$ 1,546	$ 20,070	$ 2,703
Post Modification Recorded Investment	14,639	1,535	18,908	2,682
Increase (decrease) in the allowance for loan losses as a result of modification	$ 845	$ 87	$ 983	$ 143
Commercial and industrial
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	32	15	54	23
Pre-Modification Recorded Investment	$ 36,153	$ 509	$ 47,222	$ 828
Post Modification Recorded Investment	35,971	535	46,494	2,923
Increase (decrease) in the allowance for loan losses as a result of modification	$ 13,934	$ 49	$ 13,824	$ 468
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	121	179	203	371
Pre-Modification Recorded Investment	$ 15,325	$ 20,017	$ 25,598	$ 41,085
Post Modification Recorded Investment	14,016	18,819	22,935	38,332
Increase (decrease) in the allowance for loan losses as a result of modification	$ 777	$ 1,226	$ 1,234	$ 2,240
Leasing
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	1	3	1	6
Pre-Modification Recorded Investment	$ 23	$ 122	$ 23	$ 236
Post Modification Recorded Investment	23	120	23	235
Increase (decrease) in the allowance for loan losses as a result of modification	$ 7	$ 34	$ 7	$ 66
Credit cards
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	343	311	624	596
Pre-Modification Recorded Investment	$ 3,478	$ 2,502	$ 6,404	$ 4,904
Post Modification Recorded Investment	3,503	2,757	6,804	5,400
Increase (decrease) in the allowance for loan losses as a result of modification	$ 398	$ 332	$ 852	$ 644
Home equity lines of credit
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	10	2	19	2
Pre-Modification Recorded Investment	$ 860	$ 486	$ 1,725	$ 486
Post Modification Recorded Investment	817	483	1,673	483
Increase (decrease) in the allowance for loan losses as a result of modification	$ 107	$ 13	$ 374	$ 13
Personal
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	469	250	631	517
Pre-Modification Recorded Investment	$ 7,253	$ 4,436	$ 10,325	$ 9,034
Post Modification Recorded Investment	7,251	4,443	10,321	9,038
Increase (decrease) in the allowance for loan losses as a result of modification	$ 1,720	$ 998	$ 2,730	$ 2,031
Auto
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	3	5	4	7
Pre-Modification Recorded Investment	$ 60	$ 1,965	$ 194	$ 2,001
Post Modification Recorded Investment	59	1,920	191	1,957
Increase (decrease) in the allowance for loan losses as a result of modification	$ 10	$ 348	$ 33	$ 354
Other
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Loan count | loans	13	10	21	18
Pre-Modification Recorded Investment	$ 46	$ 1,891	$ 203	$ 1,956
Post Modification Recorded Investment	46	1,891	201	1,956
Increase (decrease) in the allowance for loan losses as a result of modification	$ 5	$ 55	$ 31	$ 64