Mergers and acquisitions - Schedule of Final Allocation of Purchase Price to Assets and Liabilities Acquired in Connection with Merger (Parenthetical) (Details) $ in Thousands	Sep. 18, 2015 USD ($)
Northwest Georgia Bank
Business Acquisition [Line Items]
Net reversal allowance for loan losses	$ 4,383

Amount of reduction of allowance reserve for credit loss arising from sale of loan and lease portfolios.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=66008989&loc=d3e5212-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(d))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Name:	us-gaap_AllowanceForLoanAndLeaseLossesLoansSold
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration