Loans and allowance for loan losses - Changes in Value of Accretable Yield for PCI Loans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield Movement Schedule [Roll Forward]
Accretion	$ 7,608	$ 5,419	$ 3,538
Purchased Credit Impaired
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield Movement Schedule [Roll Forward]
Balance at the beginning of period	(17,682)	(2,444)	(1,637)
Additions through the acquisition of the Clayton Banks	0	(18,868)	0
Principal reductions and other reclassifications from nonaccretable difference	4,047	1,841	3,438
Recoveries	0	23	0
Accretion	9,010	5,299	2,631
Changes in expected cash flows	(3,868)	195	0
Balance at end of period	$ (16,587)	$ (17,682)	$ (2,444)