Mergers and acquisitions - Business Acquisition, Pro Forma Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Clayton Banks
Business Acquisition [Line Items]
Net interest income	$ 192,633	$ 171,383
Total revenues	336,404	322,045
Net income	$ 75,659	$ 64,608
Northwest Georgia Bank
Business Acquisition [Line Items]
Net interest income			$ 102,290
Total revenues			191,002
Net income			$ 46,042