Loans and allowance for loan losses - Changes in Value of Accretable Yield for PCI Loans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield Movement Schedule [Roll Forward]
Accretion			$ 3,928	$ 3,615
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	$ (14,814)	$ (16,955)	(16,587)	(17,682)
Additions through the branch acquisition of Atlantic Capital Bank	(1,167)	0	(1,167)	0
Principal reductions and other reclassifications from nonaccretable difference	30	(2,158)	250	(3,452)
Accretion	1,705	2,639	3,888	4,840
Changes in expected cash flows	(616)	(3,695)	(1,246)	(3,875)
Balance at end of period	$ (14,862)	$ (20,169)	$ (14,862)	$ (20,169)