Note 1 - Organization and Business (Details Textual) - USD ($) $ / shares in Units, $ in Thousands, shares in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash and Cash Equivalents, at Carrying Value, Ending Balance	$ 90,642	$ 137,686	$ 90,642
Cash and Cash Equivalents, Period Increase (Decrease), Total		47,100
Proceeds from Issuance of Common Stock		70,000
Proceeds from Issuance of Long-term Debt, Total				$ 48,650
Public Offering [Member]
Proceeds from Issuance of Common Stock		$ 69,987
Stock Issued During Period, Shares, New Issues		9.1
Shares Issued, Price Per Share		$ 8.25
At-the-market Offering [Member]
Proceeds from Issuance of Common Stock		$ 29,653	$ 5,026
Stock Issued During Period, Shares, New Issues	0.9	3.9
Shares Issued, Price Per Share	$ 6.06	$ 7.45	$ 6.06