Note 5 - Share-based Compensation - Stock Option Activity (Details)	9 Months Ended
	Sep. 30, 2018 $ / shares shares
Employee Stock Option [Member]
Outstanding at beginning of year (in shares) | shares	2,390,012
Outstanding at beginning of year, weighted average exercise price (in dollars per share) | $ / shares	$ 1.85
Granted (in shares) | shares
Granted, weighted average exercise price (in dollars per share) | $ / shares
Exercised (in shares) | shares	(267,055)
Exercised, weighted average exercise price (in dollars per share) | $ / shares	$ 1.30
Forfeited (in shares) | shares	(15,627)
Forfeited, weighted average exercise price (in dollars per share) | $ / shares	$ 4.34
Expired (in shares) | shares
Expired, weighted average exercise price (in dollars per share) | $ / shares
Outstanding service vesting-based at September 30, 2018 (in shares) | shares	2,107,330
Outstanding service vesting-based at September 30, 2018 (in dollars per share) | $ / shares	$ 1.90
Service vesting-based options exercisable at September 30, 2018 (in shares) | shares	1,622,980
Service vesting-based options exercisable at September 30, 2018 (in dollars per share) | $ / shares	$ 1.83
Performance Shares [Member]
Outstanding at beginning of year (in shares) | shares	999,997
Outstanding at beginning of year, weighted average exercise price (in dollars per share) | $ / shares	$ 1.64
Granted (in shares) | shares
Granted, weighted average exercise price (in dollars per share) | $ / shares
Exercised (in shares) | shares	(35,000)
Exercised, weighted average exercise price (in dollars per share) | $ / shares	$ 1.64
Outstanding service vesting-based at September 30, 2018 (in shares) | shares	964,997
Outstanding service vesting-based at September 30, 2018 (in dollars per share) | $ / shares	$ 1.64
Service vesting-based options exercisable at September 30, 2018 (in shares) | shares	465,001
Service vesting-based options exercisable at September 30, 2018 (in dollars per share) | $ / shares	$ 1.64