Note 6 - Stock-based Compensation (Details Textual) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2014	Dec. 31, 2015
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
The 2014 Employee Stock Purchase Plan [Member]
Common Stock, Capital Shares Reserved for Future Issuance	296,685		296,685		140,500	297,359
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			674
Allocated Share-based Compensation Expense	$ 16,000	$ 11,000	$ 32,000	$ 22,000
Common Stock, Capital Shares Reserved for Future Issuance	3,304,138		3,304,138			2,717,260
Allocated Share-based Compensation Expense	$ 946,000	$ 476,000	$ 1,812,000	$ 883,000