Mergers and acquisitions - Additional Information (Details)				1 Months Ended		12 Months Ended
		Feb. 08, 2017 USD ($) shares	Sep. 18, 2015 USD ($)	Jul. 31, 2017 USD ($) BankingOffice		Dec. 31, 2017 USD ($) Branch	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Business Acquisition [Line Items]
Goodwill						$ 137,190,000	$ 46,867,000	$ 46,904,000
Merger and conversion						19,034,000	3,268,000	3,543,000
Business combination, deferred taxes						$ 17,768,000	21,956,000
Business combination, bargain purchase gain								2,794,000
Core deposit intangible asset				$ 587,000
Clayton Banks
Business Acquisition [Line Items]
Business acquisition, date of acquisition agreement						Feb. 08, 2017
Acquisition purchase price		$ 236,484,000		236,484,000
Business acquisition, shares issued | shares		1,521,200
Cash purchase price		$ 184,200,000		$ 184,200,000	[1]
Number of banking offices | BankingOffice				18
Goodwill				$ 90,323,000
Merger and conversion						$ 19,034,000
Charitable contributions						$ 10,000,000
Business combination, deferred taxes				$ 0
Number of years of deductibility for income tax of the goodwill and core deposit intangible				15 years
Northwest Georgia Bank
Business Acquisition [Line Items]
Business acquisition, date of acquisition agreement						Apr. 27, 2015
Cash purchase price			$ 1,500,000
Merger and conversion							$ 3,268,000	$ 3,543,000
Business combination, deferred taxes			0
Business acquisition, effective date of acquisition						Sep. 18, 2015
Number of branches operated prior to acquisition | Branch						6
Business combination, bargain purchase gain	[2]		2,794,000
Core deposit intangible asset			$ 4,931,000
Estimated useful life of intangible assets			10 years
Deferred tax liability recorded in bargain purchase gain			$ 191,000
Number of years of deductibility for income tax of the core deposit intangible			15 years

[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.