Note 7 - Share-based Compensation - Stock Option Activity (Details) - $ / shares	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019
Share-based Payment Arrangement, Option [Member]
Outstanding (in shares)	2,043,402
Outstanding at beginning of year, weighted average exercise price (in dollars per share)	$ 1.91
Granted (in shares)	0	0
Granted, weighted average exercise price (in dollars per share)
Exercised (in shares)	(339,237)
Exercised, weighted average exercise price (in dollars per share)	$ 1.83
Forfeited (in shares)	(3,437)
Forfeited, weighted average exercise price (in dollars per share)	$ 5.69
Expired (in shares)
Expired, weighted average exercise price (in dollars per share)
Outstanding (in shares)	1,700,728
Outstanding at end of year, weighted average exercise price (in dollars per share)	$ 1.92
Stock options exercisable at year end (in shares)	1,546,830		1,546,830
Stock options exercisable at year end, weighted average exercise price (in dollars per share)			$ 1.89
Outstanding (in shares)	1,546,830
Performance Shares [Member]
Outstanding at beginning of year, weighted average exercise price (in dollars per share)	$ 1.64
Granted (in shares)
Granted, weighted average exercise price (in dollars per share)
Exercised (in shares)	(135,000)
Exercised, weighted average exercise price (in dollars per share)	$ 1.64
Outstanding at end of year, weighted average exercise price (in dollars per share)	$ 1.64
Stock options exercisable at year end (in shares)	964,997		829,997
Stock options exercisable at year end, weighted average exercise price (in dollars per share)			$ 1.64
Outstanding (in shares)	964,997
Outstanding (in shares)	829,997