Employee benefit plans - Additional Information (Details)			12 Months Ended
	Dec. 31, 2019	Jun. 28, 2016	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($)	Dec. 31, 2014 USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Discretionary contribution percentage			25.00%	25.00%
Maximum percentage of amount to be contributed by the employer			6.00%
Matching and profit sharing vesting period			5 years
Contribution provided by the bank to the plan			$ 2,211,000	$ 2,344,000	$ 1,379,000
Post retirement benefits payable			1,260,000	1,510,000
Expense related to plans			4,000	4,000	30,000
Payments to participants			191,000	191,000	205,000
Cash surrender value on bank owned life insurance			11,115,000	10,873,000
Cash value income			158,000	164,000	181,000
Stock split ratio		100
Deferred compensation			$ 3,836,000	$ 5,301,000
Number of units remaining in equity based incentive plan for cash settlement | shares			29,172,000	67,470,000
Salaries, commissions and employee benefits			$ 136,892,000	$ 130,005,000	$ 113,486,000
Deferred Compensation Plans and Agreements
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Payments to participants			1,818,000	5,163,000
Other liabilities			993,000	2,346,000
Salaries, commissions and employee benefits			$ 3,787,000	$ 3,685,000
Deferred Compensation Plans and Agreements | 2010 EBI Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation award vesting period			5 years
Employee retirement age under EBI plan			65 years
Valuation of unit by percentage of assets			7.50%
Deferred Compensation Plans and Agreements | 2012 EBI Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee retirement age under EBI plan			65 years
Fair market value per EBI | $ / shares			$ 21.4085
IPO price (USD per share) | $ / shares			19.00
Deferred Compensation Plans and Agreements | Chief Executive Officer
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
IPO price (USD per share) | $ / shares			$ 19.00
Deferred compensation						$ 3,000,000
Deferred stock units issued (in shares) | shares			157,895
Deferred Compensation Plans and Agreements | Chief Executive Officer | Scenario, Forecast
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Deferred stock units conversion basis	1-for-1