Mergers and acquisitions - Business Acquisition, Pro Forma Information (Details) - Clayton Banks - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Business Acquisition [Line Items]
Net interest income	$ 49,408	$ 42,971	$ 142,972	$ 126,115
Total revenues	88,013	88,568	250,676	243,877
Net income	$ 11,719	$ 15,402	$ 50,426	$ 49,434