Loans and allowance for loan losses - Changes in Value of Accretable Yield for PCI Loans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield Movement Schedule [Roll Forward]
Accretion	$ 8,556	$ 7,608	$ 5,419
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	(16,587)	(17,682)	(2,444)
Additions through business combinations	(1,167)	0	(18,868)
Principal reductions and other reclassifications from nonaccretable difference	61	(4,047)	(1,841)
Recoveries	0	0	(23)
Accretion	7,003	9,010	5,299
Changes in expected cash flows	(360)	(3,868)	195
Balance at end of period	$ (11,050)	$ (16,587)	$ (17,682)