Note 4 - Allowance for Loan Losses (Details Textual)	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 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	139		155
Impaired Financing Receivable Recorded Investment Below Review Threshold	$ 4,000,000		$ 4,500,000
Loans and Leases Receivable, Gross, Total	779,443,000	$ 765,284,000	770,046,000
Loans and Leases Receivable, Allowance, Ending Balance	7,155,000	7,508,000	7,977,000	$ 8,923,000
Substandard [Member]
Loans and Leases Receivable, Gross, Total	21,659,000	$ 21,932,000
Loans Not Classified as Impaired [Member] | Substandard [Member]
Loans and Leases Receivable, Gross, Total	8,730,000		12,680,000
Loans and Leases Receivable, Allowance, Ending Balance	$ 1,250,000		$ 1,490,000