Note 11 - Share Option and Equity Incentive Plan - Unvested Share Option Activity (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Number of unvested share options outstanding (in shares)	1,479,293
Number of unvested share options outstanding, weighted-average grant date fair value (in dollars per share)	$ 5.83
Number of share options granted (in shares)	796,264
Number of unvested share options granted, weighted-average grant date fair value (in dollars per share)	$ 6.15	$ 6.31	$ 4.53
Number of unvested share options vested (in shares)	(651,787)
Number of unvested share options vested, weighted-average grant date fair value (in dollars per share)	$ 5.59
Number of unvested share options forfeited (in shares)	(462,240)
Number of unvested share options forfeited, weighted-average grant date fair value (in dollars per share)	$ 6.56
Number of unvested share options outstanding (in shares)	1,161,530	1,479,293
Number of unvested share options outstanding, weighted-average grant date fair value (in dollars per share)	$ 6.06	$ 5.83