Note 6 - Embedded Derivatives and Derivative Warrant Liabilities - Schedule of Shares Issued Upon Exercise of Warrants (Details) - $ / shares		3 Months Ended
		Mar. 31, 2018		Feb. 28, 2017	Sep. 30, 2016	Apr. 30, 2016	Dec. 31, 2015	May 31, 2015	Feb. 28, 2015	Aug. 31, 2014	Dec. 31, 2013
Shares Underlying Warrants (in shares)		7,192,373	[1]	17,106,683
Shares Issues Upon Warrant Exercises (in shares)		9,094,310
2013 Warrants [Member]
Warrant, Issuance Date		Dec. 16, 2013
Warrants, Expiration date		Dec. 16, 2018
Warrant Exercise Price (in dollars per share)		$ 8.99
Shares Underlying Warrants (in shares)		55,774	[1]								71,013
Shares Issues Upon Warrant Exercises (in shares)		15,239
2014 Warrants [Member]
Warrant, Issuance Date		Aug. 05, 2014
Warrants, Expiration date		Aug. 05, 2019
Warrant Exercise Price (in dollars per share)		$ 6.83
Shares Underlying Warrants (in shares)		19,462	[1]							50,000
Shares Issues Upon Warrant Exercises (in shares)		30,538
Series A Warrants [Member]
Warrant, Issuance Date		Feb. 03, 2015
Warrants, Expiration date		Feb. 03, 2020
Warrant Exercise Price (in dollars per share)		$ 0.68
Shares Underlying Warrants (in shares)		11,417	[1]						110,833
Shares Issues Upon Warrant Exercises (in shares)		99,416
Series B Warrants [Member]
Warrant, Issuance Date		Feb. 03, 2015
Warrants, Expiration date		Aug. 03, 2015
Warrant Exercise Price (in dollars per share)	[2]
Shares Underlying Warrants (in shares)			[1]						110,883
Shares Issues Upon Warrant Exercises (in shares)		110,833
Series C Warrants [Member]
Warrant, Issuance Date		May 19, 2015
Warrants, Expiration date		May 19, 2020
Warrant Exercise Price (in dollars per share)		$ 5.50
Shares Underlying Warrants (in shares)		21,500	[1]					21,500
Shares Issues Upon Warrant Exercises (in shares)
Series D Warrants [Member]
Warrant, Issuance Date		Dec. 11, 2015
Warrants, Expiration date		Dec. 11, 2020
Warrant Exercise Price (in dollars per share)		$ 2
Shares Underlying Warrants (in shares)		930	[1]				502,500
Shares Issues Upon Warrant Exercises (in shares)		501,570
Series E Warrants [Member]
Warrant, Issuance Date		Dec. 11, 2015
Warrants, Expiration date		Dec. 11, 2020
Warrant Exercise Price (in dollars per share)	[2]
Shares Underlying Warrants (in shares)			[1]				400,000
Shares Issues Upon Warrant Exercises (in shares)		400,000
Series F Warrants [Member]
Warrant, Issuance Date		Apr. 01, 2016
Warrants, Expiration date		Apr. 01, 2021
Warrant Exercise Price (in dollars per share)		$ 2
Shares Underlying Warrants (in shares)		280,787	[1]			514,644
Shares Issues Upon Warrant Exercises (in shares)		233,857
Series G Warrants [Member]
Warrant, Issuance Date		Apr. 01, 2016
Warrants, Expiration date		Apr. 01, 2017
Warrant Exercise Price (in dollars per share)	[2]
Shares Underlying Warrants (in shares)			[1]			328,571
Shares Issues Upon Warrant Exercises (in shares)		328,571
Series H Warrants [Member]
Warrant, Issuance Date		Apr. 01, 2016
Warrants, Expiration date		Oct. 01, 2016
Warrant Exercise Price (in dollars per share)	[2]
Shares Underlying Warrants (in shares)			[1]			1,029,286
Shares Issues Upon Warrant Exercises (in shares)		1,029,286
Series I Warrants [Member]
Warrant, Issuance Date		Sep. 13, 2016
Warrants, Expiration date		Sep. 13, 2021
Warrant Exercise Price (in dollars per share)		$ 11
Shares Underlying Warrants (in shares)		712,503	[1]		712,503
Shares Issues Upon Warrant Exercises (in shares)
Series J Warrants [Member]
Warrant, Issuance Date		Sep. 13, 2016
Warrants, Expiration date		Sep. 13, 2017
Warrant Exercise Price (in dollars per share)	[2]
Shares Underlying Warrants (in shares)			[1]		185,000
Shares Issues Upon Warrant Exercises (in shares)		185,000
Series K Warrants [Member]
Warrant, Issuance Date		Feb. 17, 2017
Warrants, Expiration date		Feb. 17, 2022
Warrant Exercise Price (in dollars per share)		$ 0.60
Shares Underlying Warrants (in shares)		6,090,000	[1]	6,250,000
Shares Issues Upon Warrant Exercises (in shares)		160,000
Series L Warrants [Member]
Warrant, Issuance Date		Feb. 17, 2017
Warrants, Expiration date		Feb. 17, 2018
Warrant Exercise Price (in dollars per share)	[2]
Shares Underlying Warrants (in shares)			[1]	570,000
Shares Issues Upon Warrant Exercises (in shares)		570,000
Series M-A Warrants [Member]
Warrant, Issuance Date		Feb. 17, 2017
Warrants, Expiration date		Nov. 17, 2017
Warrant Exercise Price (in dollars per share)	[2],[3]
Shares Underlying Warrants (in shares)			[1]	2,305,000
Shares Issues Upon Warrant Exercises (in shares)		1,485,000
Series M-B Warrants [Member]
Warrant, Issuance Date		Feb. 17, 2017
Warrants, Expiration date		Nov. 17, 2017
Warrant Exercise Price (in dollars per share)	[2],[4]
Shares Underlying Warrants (in shares)			[1]	3,945,000
Shares Issues Upon Warrant Exercises (in shares)		3,945,000

[1] This table does not include 1,393 equity-classified warrants issued between 2008 through 2012, with strike prices between $17.70 and $24.45 per share.

[2] Warrants have either been fully exercised and/or expired as of March 31, 2018.

[3] In October 2017, 1,485,000 Series M warrants were repriced between $0.60 and $0.65 per warrant. Of those warrants that were repriced, all were exercised in the fourth quarter of 2017, providing proceeds of $1.0 million.

[4] In September 2017, 3,945,000 Series M warrants were repriced to $0.60. Of those warrants that were repriced, all were exercised in the second half of 2017, providing proceeds of $2.4 million.