Note 1 - The Company and Basis of Presentation (Details Textual) - USD ($)			9 Months Ended
	Mar. 22, 2017	Jun. 17, 2016	Sep. 30, 2017	Sep. 30, 2016	May 22, 2017	Dec. 31, 2016		May 10, 2016	Dec. 31, 2015
Proceeds from Issuance of Common Stock			$ 31,440,000	$ 13,886,000
Common Stock, Par or Stated Value Per Share			$ 0.0001			$ 0.0001		$ 0.0001
Retained Earnings (Accumulated Deficit)			$ (95,357,000)			$ (68,622,000)	[1]
Cash and Cash Equivalents, at Carrying Value			19,207,000	$ 14,011,000		$ 8,086,000	[1]		$ 7,360,000
The 2017 Loan Agreement [Member] | CRG LP [Member]
Debt Agreement, Covenant, Additional Funding			$ 10,000,000		$ 10,000,000
March 2017 Offering [Member]
Stock Issued During Period, Shares, New Issues	8,625,000
Share Price	$ 4
Proceeds from Issuance of Common Stock	$ 34,500,000
Proceeds From Issuance of Common Stock, Net	$ 31,440,000
June 2016 Offering [Member]
Stock Issued During Period, Shares, New Issues		3,105,000
Share Price		$ 5
Proceeds from Issuance of Common Stock		$ 15,525,000
Proceeds From Issuance of Common Stock, Net		$ 13,886,000

[1] The condensed consolidated balance sheet as of December 31, 2016 has been derived from the audited consolidated financial statements as of that date.