Note 5 - Share-based Compensation - Stock Option Activity (Details)	6 Months Ended
	Jun. 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	(142,914)
Exercised, weighted average exercise price (in dollars per share) | $ / shares	$ 1.19
Forfeited (in shares) | shares	(8,022)
Forfeited, weighted average exercise price (in dollars per share) | $ / shares	$ 4.30
Expired (in shares) | shares
Expired, weighted average exercise price (in dollars per share) | $ / shares
Outstanding service vesting-based at June 30, 2018 (in shares) | shares	2,239,076
Outstanding service vesting-based at June 30, 2018 (in dollars per share) | $ / shares	$ 1.89
Options exercisable at June 30, 2018 (in shares) | shares	1,652,714
Options exercisable at June 30, 2018 (in dollars per share) | $ / shares	$ 1.80
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 June 30, 2018 (in shares) | shares	964,997
Outstanding service vesting-based at June 30, 2018 (in dollars per share) | $ / shares	$ 1.64
Options exercisable at June 30, 2018 (in shares) | shares	465,001
Options exercisable at June 30, 2018 (in dollars per share) | $ / shares	$ 1.64