Mergers and acquisitions - Business Acquisition, Pro Forma Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Atlantic Capital Bank
Business Acquisition [Line Items]
Net interest income	$ 229,607	$ 220,269
Total revenues	365,794	354,258
Net income	$ 79,923	$ 78,762
Clayton Bank And Trust And American City Bank
Business Acquisition [Line Items]
Net interest income			$ 192,633
Total revenues			336,404
Net income			$ 75,659