Loans and allowance for loan losses - Changes in Value of Accretable Yield for PCI Loans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield Movement Schedule [Roll Forward]
Accretion			$ 6,030	$ 5,745
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	$ (14,862)	$ (20,169)	(16,587)	(17,682)
Additions through the branch acquisition of Atlantic Capital Bank	0	0	(1,167)	0
Principal reductions and other reclassifications from nonaccretable difference	(150)	(84)	100	(3,536)
Accretion	1,583	2,103	5,471	6,943
Changes in expected cash flows	110	6	(1,136)	(3,869)
Balance at end of period	$ (13,319)	$ (18,144)	$ (13,319)	$ (18,144)