Note 10 - Stockholders' Equity (Details Textual) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	1 Months Ended	3 Months Ended		12 Months Ended
	Oct. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Common Stock, Shares Authorized				160,000	160,000
Common Stock, Par or Stated Value Per Share				$ 0.0013	$ 0.0013
Preferred Stock, Shares Authorized				20,000	20,000
Preferred Stock, Par or Stated Value Per Share				$ 0.001	$ 0.001
Replacement Shelf Registration	$ 250,000		$ 250,000
Common Stock Sales Agreement with Agent, Maximum Aggregate Offering Price	$ 75,000		$ 75,000
Common Stock, Shares Subscribed but Unissued		300				300
Common Stock, Share Subscribed but Unissued, Subscriptions Receivable		$ 2,100				$ 2,100
Sale of Subscribed Common Stock, Price Per Share		$ 6.79				$ 6.79
Public Offering [Member]
Proceeds from Issuance of Common Stock					$ 69,987
Stock Issued During Period, Shares, New Issues					9,100
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		900			3,900
Shares Issued, Price Per Share		$ 6.06			$ 7.45	$ 6.06
Payments of Stock Issuance Costs		$ 5,000			$ 27,500