Mergers and acquisitions - Schedule of Final Allocation of Purchase Price to Assets and Liabilities Acquired in Connection with Merger (Details) - USD ($) $ in Thousands		12 Months Ended
	Sep. 18, 2015	Dec. 31, 2015
Increase (decrease) to net assets as a result of fair value adjustments to assets acquired and liabilities assumed:
Bargain purchase gain		$ 2,794
Northwest Georgia Bank
Final Allocation of Purchase Price for Northwest Georgia Bank:
Total cash purchase price	$ 1,500
Net assets acquired:
Stockholders' equity at September 18, 2015	6,478
Increase (decrease) to net assets as a result of fair value adjustments to assets acquired and liabilities assumed:
Securities	535
Loans, net of the reversal of NWGB's allowance for loan losses of $4,383	(8,024)
Premises and equipment	3,731
Core deposits intangible	4,931
Foreclosed real estate	(1,922)
Other assets	(264)
Deposits	(302)
FHLB funds purchased	(378)
Other liabilities	(491)
Total net fair value adjustments	4,294
Bargain purchase gain	$ 2,794