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	Sep. 18, 2015	Jul. 31, 2017		Dec. 31, 2017	Dec. 31, 2015	Dec. 31, 2016
Equity consideration
Total equity consideration						$ 52,284
Allocation of consideration paid:
Goodwill						$ 137,190	$ 46,904	$ 46,867
Total consideration paid				$ 236,484
Bargain purchase gain							$ 2,794
Clayton Banks
Equity consideration
Common stock issued		1,521,200
Price per share as of July 31, 2017				$ 34.37
Total equity consideration				$ 52,284
Cash consideration		$ 184,200		184,200	[1]
Total consideration paid		$ 236,484		236,484
Allocation of consideration paid:
Fair value of net assets acquired including identifiable intangible assets				146,161
Goodwill				$ 90,323
Clayton Banks | Common Stock
Equity consideration
Common stock issued				1,521,200
Northwest Georgia Bank
Equity consideration
Cash consideration			$ 1,500
Allocation of consideration paid:
Fair value of net assets acquired including identifiable intangible assets			4,294
Bargain purchase gain	[2]		$ 2,794

[1] Amount was deposited into an interest-bearing deposit account with the Bank in the name of the Seller as of July, 31, 2017.

[2] The bargain purchase gain resulting from the merger has been recognized in the Banking operating segment during the year ended December 31, 2015.