Mergers and acquisitions - Additional Information (Details)					3 Months Ended		9 Months Ended
	Jul. 31, 2017 USD ($)		Feb. 08, 2017 USD ($) shares	Sep. 18, 2015 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) Branch	Sep. 30, 2016 USD ($)	Jul. 30, 2017 BankingOffice	Dec. 31, 2016 USD ($)
Business Acquisition [Line Items]
Goodwill					$ 138,910,000	$ 46,867,000	$ 138,910,000	$ 46,867,000		$ 46,867,000
Merger and conversion					15,711,000	1,122,000	16,965,000	3,268,000
Core deposit intangible asset					587,000		$ 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	[1]	$ 184,200,000
Number of banking offices | BankingOffice									18
Goodwill	92,043,000
Merger and conversion					$ 15,711,000		$ 16,965,000
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						$ 1,122,000		$ 3,268,000
Business acquisition, effective date of acquisition							Sep. 18, 2015
Number of branches operated prior to acquisition | Branch							6
Business combination, bargain purchase gain				2,794,000
Core deposit intangible asset				$ 4,931,000

[1] Amount was deposited into an interest-bearing account with the Bank of which $34,200 was included in cash and cash equivalents and interest-bearing deposits in the above schedule of net assets acquired as of July, 31, 2017