Note 7 - Stock-based Compensation (Details Textual) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2014
Common Stock, Capital Shares Reserved for Future Issuance	3,834,918		3,834,918		3,275,291
Allocated Share-based Compensation Expense	$ 747,000	$ 687,000	$ 2,459,000	$ 2,499,000
The 2014 Employee Stock Purchase Plan [Member]
Common Stock, Capital Shares Reserved for Future Issuance	247,774		247,774		292,821	140,500
Share-based Compensation Arrangement by Share-based Payment Award Number of Annual Additional Shares Percentage	1.00%		1.00%
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	304,991		304,991
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period	0		45,047
Allocated Share-based Compensation Expense	$ 18,000	$ 30,000	$ 100,000	$ 62,000
The 2014 Employee Stock Purchase Plan [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized			281,000