Note 7 - Stock-based Compensation (Details Textual) - USD ($)		3 Months Ended		6 Months Ended
	Jan. 01, 2019	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2014
Share-based Payment Arrangement, Expense		$ 101,000	$ 922,000	$ 881,000	$ 1,743,000
The 2014 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized		192,436		192,436			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, Maximum Number of Additional Shares Authorizable	281,000
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period		0	51,984	0	51,984
Share-based Payment Arrangement, Expense		$ 17,000	$ 24,000	$ 68,000	$ 48,000