Note 4 - Allowance for Loan Losses (Details Textual)	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Loan Balance For Reviewing High Risk Loans Minimum	$ 250,000	$ 250,000
Impaired Financing Receivable, Number of Loans Below Review Threshold	125		153
Impaired Financing Receivable Recorded Investment Below Review Threshold	$ 3,100,000	4,300,000
Loans and Leases Receivable, Gross, Total	766,796,000	765,284,000	$ 775,566,000
Loans and Leases Receivable, Allowance, Ending Balance	7,159,000	7,508,000	8,043,000	$ 8,923,000
Substandard [Member]
Loans and Leases Receivable, Gross, Total	20,785,000	$ 21,932,000
Loans Not Classified as Impaired [Member] | Substandard [Member]
Loans and Leases Receivable, Gross, Total	9,550,000		13,040,000
Loans and Leases Receivable, Allowance, Ending Balance	$ 1,300,000		$ 1,190,000