Note 6 - Stock-based Compensation (Details Textual) - USD ($)		3 Months Ended		9 Months Ended
	Jan. 01, 2019	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2014
Share-based Payment Arrangement, Expense		$ 96,000	$ 904,000	$ 977,000	$ 2,647,000
The 2014 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period			0		51,984
Share-based Payment Arrangement, Expense		$ 0	$ 20,000	$ 68,000	$ 100,000
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		0