Note 15 - Stock-based Compensation - Stock Purchase Plan (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Issuance of shares of common stock as a result of stock purchased under the Employee Stock Purchase Plan (in shares)	15,528
Employee Stock Purchase Plan
Shares granted (in shares)	17,305	12,414	18,711
Issuance of shares of common stock as a result of stock purchased under the Employee Stock Purchase Plan (in shares)	15,528	13,318	20,192
Weighted average fair value per share granted (in dollars per share)	$ 6.63	$ 6.42	$ 3.28