Note 7 - Share-based Compensation - Stock Option Activity (Details)	6 Months Ended
	Jun. 30, 2020 $ / shares shares	Jun. 30, 2019 shares
Share-based Payment Arrangement, Option [Member]
Outstanding (in shares) | shares	1,570,455
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 1.96
Granted (in shares) | shares	0	0
Granted, weighted average exercise price (in dollars per share) | $ / shares	$ 0
Exercised (in shares) | shares	(393,680)
Exercised, weighted average exercise price (in dollars per share) | $ / shares	$ 1.92
Forfeited (in shares) | shares	0
Forfeited, weighted average exercise price (in dollars per share) | $ / shares	$ 0
Expired (in shares) | shares	0
Expired, weighted average exercise price (in dollars per share) | $ / shares	$ 0
Outstanding (in shares) | shares	1,176,775
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 1.98
Options exercisable (in shares) | shares	1,148,338
Options exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 1.94
Performance Shares [Member]
Outstanding (in shares) | shares	737,497
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 1.64
Granted (in shares) | shares	0
Granted, weighted average exercise price (in dollars per share) | $ / shares	$ 0
Exercised (in shares) | shares	(17,113)
Exercised, weighted average exercise price (in dollars per share) | $ / shares	$ 1.64
Outstanding (in shares) | shares	720,384
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 1.64
Options exercisable (in shares) | shares	720,384
Options exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 1.64