Note 5 - Share-based Compensation - Stock Options Issued Under the 2007 Equity Incentive Plan (Details) - The 2007 Equity Incentive Plan [Member] - USD ($)		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Options outstanding, balance (in shares)		9,691,586	9,831,586
Weighted average exercise price per share, balance (in dollars per share)		$ 0.42	$ 0.44
Options outstanding, expired (in shares)		(870,000)	(50,000)
Exercise price per share, expired (in dollars per share)		$ 0.57	$ 1.89
Weighted average price per share, expired (in dollars per share)		$ 0.57	$ 0.91
Options outstanding, balance (in shares)		8,821,586	9,781,586
Weighted average exercise price per share, balance (in dollars per share)		$ 0.41	$ 0.43
Aggregate intrinsic value, balance	[1]
Minimum [Member]
Exercise price per share, balance (in dollars per share)		$ 0.23	0.23
Exercise price per share, balance (in dollars per share)		0.23	0.23
Maximum [Member]
Exercise price per share, balance (in dollars per share)		0.94	1.89
Exercise price per share, balance (in dollars per share)		$ 1.65	$ 1.65

[1] Aggregate intrinsic value based on closing common stock price of $0.16 at March 31, 2020.