Note 6 - Share-based Compensation - Stock Option Activity (Details) - $ / shares	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 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)	(236,061)
Exercised, weighted average exercise price (in dollars per share)	$ 1.96
Forfeited (in shares)	(3,438)
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)	1,803,903
Outstanding at end of year, weighted average exercise price (in dollars per share)	$ 1.90
Stock options exercisable at year end (in shares)	1,600,799		1,600,799
Stock options exercisable at year end, weighted average exercise price (in dollars per share)			$ 1.87
Outstanding (in shares)	1,600,799
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)
Exercised, weighted average exercise price (in dollars per share)
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		964,997
Stock options exercisable at year end, weighted average exercise price (in dollars per share)			$ 1.64
Outstanding (in shares)	964,997
Outstanding (in shares)	964,997