Mergers and acquisitions - Schedule of Consideration Paid and Allocation of Purchase Price to Net Assets Acquired (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended	12 Months Ended
	Feb. 08, 2017	Jul. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Equity consideration
Total equity consideration			$ 0	$ 52,284	$ 0
Preliminary allocation of consideration paid:
Goodwill			$ 137,190	$ 137,190	$ 46,867
Clayton Bank and Trust and American City Bank
Equity consideration
Common stock issued (in shares)	1,521,200
Price per share as of July 31, 2017 (in dollars per share)		$ 34.37
Total equity consideration		$ 52,284
Cash consideration	$ 184,200	184,200
Total consideration paid	$ 236,484	236,484
Preliminary allocation of consideration paid:
Fair value of net assets acquired including identifiable intangible assets		146,161
Goodwill		90,323
Total consideration paid		$ 236,484
Clayton Bank and Trust and American City Bank | Common stock
Equity consideration
Common stock issued (in shares)		1,521,200