Note 15 - Stock-based Compensation - Stock Purchase Plan (Details) - Employee Stock Purchase Plan [Member] - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Shares granted (in shares)	12,414	18,711	23,408
Issuance of shares of common stock as a result of stock purchased under the Employee Stock Purchase Plan (in shares)	13,318	20,192	24,033
Weighted average fair value per share granted (in dollars per share)	$ 6.42	$ 3.28	$ 2.39