Employee benefit plans - Additional Information (Details)		12 Months Ended
	Jun. 28, 2016	Dec. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($)	Dec. 31, 2014 USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Discretionary contribution percentage		50.00%	25.00%	25.00%
Maximum percentage of amount to be contributed by the employer		6.00%	6.00%
Matching and profit sharing vesting period		3 years	5 years	5 years
Contribution provided by the bank to the plan		$ 2,325,000	$ 2,211,000	$ 2,344,000
Post retirement benefits payable		1,315,000	1,260,000
Expense related to plans		1,000	4,000	4,000
Payments to participants		150,000	191,000	191,000
Cash surrender value on bank owned life insurance		11,357,000	11,115,000
Cash value income		240,000	158,000	164,000
Deferred compensation		1,718,000	$ 3,836,000
Number of units remaining in equity based incentive plan for cash settlement | shares			29,172,000
Salaries, commissions and employee benefits		152,084,000	$ 136,892,000	130,005,000
Deferred Compensation Plans and Agreements
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Payments to participants		1,191,000	1,818,000	5,163,000
Other liabilities			993,000
Salaries, commissions and employee benefits		484,000	$ 3,787,000	$ 3,685,000
Deferred Compensation Plans and Agreements | 2012 EBI Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock split ratio	100
Deferred Compensation Plans and Agreements | Chief Executive Officer
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Deferred compensation		$ 3,000,000			$ 3,000,000
Deferred stock units issued (in shares) | shares		157,895
IPO price (USD per share) | $ / shares		$ 19.00
Deferred stock units conversion basis		1-for-1