Note 8 - Stockholders' Equity (Details Textual) - USD ($)		1 Months Ended				3 Months Ended			9 Months Ended
	Feb. 14, 2018	Jul. 30, 2021	Dec. 31, 2020	Dec. 31, 2019	Jan. 31, 2019	Sep. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Feb. 16, 2018
Preferred Stock, Shares Authorized (in shares)			5,000,000			5,000,000			5,000,000
Preferred Stock, Par or Stated Value Per Share (in dollars per share)			$ 0.001			$ 0.001			$ 0.001
Preferred Stock, Shares Issued, Total (in shares)			52,369			52,276			52,276
Preferred Stock, Shares Outstanding, Ending Balance (in shares)			52,369			52,276			52,276
Common Stock, Shares Authorized (in shares)			100,000,000			100,000,000			100,000,000
Common Stock, Par or Stated Value Per Share (in dollars per share)			$ 0.001			$ 0.001			$ 0.001
Common Stock, Shares, Issued, Total (in shares)			84,926,129			95,562,955			95,562,955
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)			2,753,999			2,645,690			2,645,690
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross (in shares)									0	0
Share-based Payment Arrangement, Expense						$ 239,000		$ 384,000	$ 960,000	$ 1,160,000
Share Price (in dollars per share)						$ 0.74			$ 0.74
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period (in shares)									0	0
Share-based Payment Arrangement, Option [Member]
Share-based Payment Arrangement, Expense						$ 0		9,000	$ 4,000	$ 51,000
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount, Total						0			0
Share-based Payment Arrangement, Expense, Tax Benefit						0		0	0	0
Restricted Stock Units (RSUs) [Member]
Share-based Payment Arrangement, Expense						200,000		400,000	1,000,000.0	1,100,000
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount, Total						$ 200,000			$ 200,000
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition (Year)									1 year
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number, Ending Balance (in shares)			412,642			204,452			204,452
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Fair Value						$ 200,000			$ 200,000
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value									$ 300,000	100,000
The 2015 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant (in shares)						167,429			167,429
Officer and Director Share Purchase Plan [Member]
Common Stock, Shares, Issued, Total (in shares)						0			0
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant (in shares)						92,170			92,170
Series B Financing Warrants [Member]
Class Of Warrant Or Right Warrants Expired (in shares)		1,083,091
Class of Warrant or Right, Number of Securities Called By Warrants or Rights Expired (in shares)		108,309
Conversion of Series B Preferred Stock into Common Stock [Member]
Conversion of Stock, Shares Converted (in shares)							93
Conversion of Stock, Shares Issued (in shares)							372,000
Series A Preferred Stock [Member]
Preferred Stock, Shares Outstanding, Ending Balance (in shares)						52,191			52,191
Dividends, Preferred Stock, Total						$ 1,000,000.0		$ 967,000	$ 3,100,000	$ 2,900,000
Series A Preferred Stock [Member] | CRG [Member]
Preferred Stock Dividends, Shares (in shares)			3,866	3,580	2,945
Series A Preferred Stock [Member] | Conversion of Principal Amount of Senior Secured Loan to Newly Authorized Series A Preferred Stock [Member]
Preferred Stock, Dividend Rate, Percentage	8.00%
Series B Preferred Stock [Member]
Preferred Stock, Par or Stated Value Per Share (in dollars per share)											$ 0.001
Preferred Stock, Shares Outstanding, Ending Balance (in shares)			178			85			85
Preferred Stock, Convertible, Conversion Price (in dollars per share)			$ 0.25