Note 10 - Stockholders' Equity (Details Textual) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended	6 Months Ended
	Jan. 31, 2017	Jun. 30, 2018	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Mar. 31, 2017
Common Stock, Shares Authorized		160,000,000	160,000,000		160,000,000
Common Stock, Par or Stated Value Per Share		$ 0.0013	$ 0.0013		$ 0.0013
Preferred Stock, Shares Authorized		20,000,000	20,000,000		20,000,000
Preferred Stock, Par or Stated Value Per Share		$ 0.001	$ 0.001		$ 0.001
Replacement Shelf Registration						$ 250,000
Common Stock Sales Agreement with Agent, Maximum Aggregate Offering Price	$ 75,000
Stock Issued During Period, Shares, New Issues		1,993,921
Stock Issued During Period, Value, New Issues		$ 14,500	$ 24,815
Sale of Stock, Price Per Share		$ 7.71	$ 7.71
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax			$ 0	$ 0