Note 9 - Stockholders' Equity (Details Textual)							1 Months Ended		3 Months Ended	6 Months Ended	25 Months Ended	50 Months Ended	52 Months Ended
	May 26, 2017	Feb. 14, 2017 USD ($) $ / shares shares	Dec. 20, 2016 USD ($) $ / shares shares	Jun. 13, 2016 USD ($) $ / shares shares	Oct. 02, 2015 USD ($)	Feb. 01, 2013 USD ($)	Sep. 30, 2015 USD ($)	Feb. 25, 2013 USD ($)	Mar. 31, 2017 USD ($)	Jun. 30, 2017 USD ($) $ / shares shares	Jun. 30, 2017 USD ($) $ / shares shares	Mar. 31, 2017 USD ($) shares	Jun. 30, 2017 USD ($) $ / shares shares	Jun. 26, 2017 USD ($) $ / shares shares	Jun. 09, 2017 USD ($) $ / shares shares	May 16, 2017 shares	May 15, 2017 shares	Dec. 31, 2016 $ / shares shares	Jun. 30, 2016 shares	May 31, 2016 shares	May 27, 2015 $ / shares shares	Jan. 15, 2014 $ / shares shares	Feb. 22, 2013 $ / shares shares
Shelf Registration Statement Amount | $							$ 75,000,000
Common Stock, Shares Authorized										112,500,000	112,500,000		112,500,000					112,500,000	112,500,000	75,000,000
Proceeds from Warrant Exercises | $										$ 5,100,000	$ 800,000		$ 2,100,000
Class of Warrant or Right, Number of Securities Called by Warrants or Rights														506,630	790,411
Preferred Stock, Shares Authorized										100,000	100,000		100,000					100,000	100,000
Aggregate of Common and Preferred Stock, Shares Authorized																			112,600,000
Common Stock, Shares, Outstanding										5,914,226	5,914,226		5,914,226			4,070,172	56,982,418	2,230,118
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number										680,942	680,942		680,942			200,000	2,400,000	210,023
Class of Warrant or Right, Outstanding										599,855	599,855		599,855			2,400,000	33,500,000	1,487,958
Aggregate Offering Price, Additions | $					$ 7,500,000
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares										$ 21.22	$ 21.22		$ 21.22	$ 1.65	$ 2.70			$ 9.39
Common Stock, Par or Stated Value Per Share | $ / shares										$ 0.01	$ 0.01		$ 0.01					$ 0.01
2013 Warrants, 2014 Warrants, and June 2016 Warrants [Member]
Increase (Decrease) in Fair Value of Warrants | $															$ 200,000
2015 Warrants [Member]
Increase (Decrease) in Fair Value of Warrants | $														$ 100,000
February 14, 2017 Public Offering [Member]
Stock Issued During Period, Shares, New Issues		1,384,704
Shares Issued, Price Per Share | $ / shares		$ 3.22
Maximum Beneficial Ownership of the Purchaser		9.99%
Proceeds from Issuance of Common Stock | $		$ 5,000,000
February 14, 2017 Public Offering [Member] | Series AA Warrant [Member]
Class of Warrant or Right, Term		5 years
Stock Issued During Period, Shares, Exercise of Warrants										636,713
Proceeds from Warrant Exercises | $										$ 2,100,000
Class of Warrant or Right, Number of Securities Called by Warrants or Rights		1,177,790
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares		$ 3.22
February 14, 2017 Public Offering [Member] | Pre-Funded Series BB Warrant [Member]
Stock Issued During Period, Shares, Exercise of Warrants										185,713
Class of Warrant or Right, Number of Securities Called by Warrants or Rights		185,713
Shares Issued, Price Per Share | $ / shares		$ 3.08
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares		$ 0.01
December 2016 Common Stock Offering [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights			367,343
Warrants Exercisable Period			180 days
Warrants Issued, Expiration Period			5 years 182 days
Stock Issued During Period, Shares, New Issues			367,346
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares			$ 6.44
Share Price | $ / shares			$ 4.90
Proceeds from Issuance of Common Stock | $			$ 1,800,000
June 2016 Common Stock Offering [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights				630,252
Stock Issued During Period, Shares, New Issues				165,126
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares				$ 19.60
Common Stock, Par or Stated Value Per Share | $ / shares				0.01
Share Price | $ / shares				$ 19.04
Proceeds from Issuance of Common Stock | $				$ 6,000,000
June 2016 Common Stock Offering [Member] | Series A Warrant [Member]
Warrants Exercisable Period				180 days
Warrants Issued, Expiration Period				5 years 182 days
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right				0.5
June 2016 Common Stock Offering [Member] | Series C Warrant [Member]
Warrants Exercisable Period				180 days
Warrants Issued, Expiration Period				1 year
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right				1
June 2016 Common Stock Offering [Member] | Series D Warrant [Member]
Warrants Exercisable Period				180 days
Warrants Issued, Expiration Period				5 years 182 days
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right				0.5
February 2013 Preferred Stock Offering [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights																							95,811
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares																							$ 74.34
January 2014 Common Stock Offering [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights																						64,348
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares																						$ 57.40
May 2015 Common Stock Offering [Member] | 2015 Warrants [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights																					139,284
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares																					$ 36.40
ATM Agreement [Member]
Commission on Proceeds from ATM Shares Percent								3.00%
Legal Fees | $								$ 50,000
Aggregate Offering Price, Additions | $					$ 7,500,000
Aggregate Offering Price | $						$ 25,000,000			$ 17,400,000
Stock Issued During Period, Shares, New Issues												1,479,535
Proceeds from Issuance of Common Stock | $												$ 7,600,000
Reverse Stock Split [Member]
Stockholders' Equity Note, Stock Split, Conversion Ratio	14