Mergers and acquisitions - Additional Information (Details)				3 Months Ended		9 Months Ended
	Jul. 31, 2017 USD ($)		Feb. 08, 2017 USD ($) shares	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Jul. 30, 2017 BankingOffice
Business Acquisition [Line Items]
Goodwill				$ 137,190,000	$ 138,910,000	$ 137,190,000	$ 138,910,000	$ 137,190,000
Merger and conversion					15,711,000	1,193,000	16,965,000
Business combination, deferred taxes				21,293,000		$ 21,293,000		$ 17,768,000
Clayton Banks
Business Acquisition [Line Items]
Business acquisition, date of acquisition agreement						Feb. 08, 2017
Acquisition purchase price	$ 236,484,000		$ 236,484,000
Business acquisition, shares issued | shares			1,521,200
Cash purchase price	184,200,000	[1]	$ 184,200,000
Number of banking offices | BankingOffice									18
Goodwill	90,323,000
Merger and conversion				$ 0	$ 15,711,000	$ 1,193,000	$ 16,965,000
Business combination, deferred taxes	$ 0
Number of years of deductibility for income tax of goodwill and intangibles	15 years

[1] Amount was deposited into an interest-bearing account with the Bank in the name of the Seller as of July 31, 2017.