Note 12 - Subsequent Event (Details Textual) - Private Placement [Member] - Casdin Capital LLC [Member] - Subsequent Event [Member] $ / shares in Units, $ in Millions	Aug. 09, 2018 USD ($) $ / shares shares
Proceeds from Issuance of Common Stock | $	$ 20
Stock Issued During Period, Shares, New Issues | shares	1,428,571
Shares Issued, Price Per Share | $ / shares	$ 14