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
	Apr. 05, 2019	Feb. 08, 2017	Jul. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Purchase price:
Total equity consideration				$ 0	$ 0	$ 52,284
Preliminary allocation of consideration:
Goodwill				$ 169,051	$ 137,190	$ 137,190
Atlantic Capital Bank
Purchase price:
Deposit premium	$ 36,790
Preliminary allocation of consideration:
Fair value of net assets acquired including identifiable intangible assets	4,829
Goodwill	31,961
Total consideration paid	$ 36,790
Clayton Bank And Trust And American City Bank
Purchase price:
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:
Fair value of net assets acquired including identifiable intangible assets			146,161
Goodwill		$ 90,323	90,323
Total consideration paid			$ 236,484
Common stock | Clayton Bank And Trust And American City Bank
Purchase price:
Common stock issued (in shares)			1,521,200