Mergers and acquisitions - Clayton Bank and Trust and American City Bank Narrative (Details) $ in Thousands		1 Months Ended	12 Months Ended
	Feb. 08, 2017 USD ($) bank_location shares	Jul. 31, 2017 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Goodwill			$ 169,051	$ 137,190	$ 137,190
Merger costs			$ 5,385	$ 1,594	19,034
Clayton Bank And Trust And American City Bank
Business Acquisition [Line Items]
Total consideration paid	$ 236,484	$ 236,484
Business acquisition, shares issued (in shares) | shares	1,521,200
Cash consideration	$ 184,200	184,200
Number of banking offices | bank_location	18
Goodwill	$ 90,323	$ 90,323
Merger costs					19,034
Business Combination, Charitable Foundation					$ 10,000