Note 11 - Share Option and Equity Incentive Plan - Restricted Stock Unit Activity (Details) - Restricted Stock Units (RSUs) [Member]	12 Months Ended
	Dec. 31, 2018 $ / shares shares
Unvested balance (in shares) | shares	418,518
Unvested balance, weighted average exercise price (in dollars per share) | $ / shares	$ 14.93
Granted (in shares) | shares	166,008
Granted, weighted average exercise price (in dollars per share) | $ / shares	$ 13.37
Cancelled (in shares) | shares	(112,694)
Cancelled, weighted average exercise price (in dollars per share) | $ / shares	$ 13.09
Vested (in shares) | shares	(170,878)
Vested, weighted average exercise price (in dollars per share) | $ / shares	$ 16.98
Unvested balance (in shares) | shares	300,954
Unvested balance, weighted average exercise price (in dollars per share) | $ / shares	$ 13.88