v3.8.0.1
Note 12 - Stock-based Compensation - Stock Options Outstanding, Vested and Exercisable (Details) - $ / shares
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Exercise Price Range, Lower Range Limit (in dollars per share)  
Exercise Price Range, Upper Range Limit (in dollars per share)  
Number of Stock Options Outstanding (in shares) 8,455,098 6,307,756
Weighted-average Remaining Contractual LIfe (Year) 7 years 36 days  
Options Outstanding, Weighted-average Exercise Price Per Share (in dollars per share) $ 4.25 $ 5
Shares Subject to Stock Options (in shares) 4,529,174  
Options Vested and Exercisable, Weighted-average Exercise Price Per Share (in dollars per share) $ 5.11  
Exercise Price Range 1 [Member]    
Exercise Price Range, Lower Range Limit (in dollars per share) 1.20  
Exercise Price Range, Upper Range Limit (in dollars per share) $ 2.30  
Number of Stock Options Outstanding (in shares) 105,900  
Weighted-average Remaining Contractual LIfe (Year) 8 years 146 days  
Options Outstanding, Weighted-average Exercise Price Per Share (in dollars per share) $ 2.13  
Shares Subject to Stock Options (in shares) 38,400  
Options Vested and Exercisable, Weighted-average Exercise Price Per Share (in dollars per share) $ 1.84  
Exercise Price Range 2 [Member]    
Exercise Price Range, Lower Range Limit (in dollars per share) 2.40  
Exercise Price Range, Upper Range Limit (in dollars per share) $ 3.77  
Number of Stock Options Outstanding (in shares) 5,592,633  
Weighted-average Remaining Contractual LIfe (Year) 7 years 255 days  
Options Outstanding, Weighted-average Exercise Price Per Share (in dollars per share) $ 3.10  
Shares Subject to Stock Options (in shares) 1,948,300  
Options Vested and Exercisable, Weighted-average Exercise Price Per Share (in dollars per share) $ 3.17  
Exercise Price Range 3 [Member]    
Exercise Price Range, Lower Range Limit (in dollars per share) 3.92  
Exercise Price Range, Upper Range Limit (in dollars per share) $ 6.60  
Number of Stock Options Outstanding (in shares) 2,060,565  
Weighted-average Remaining Contractual LIfe (Year) 5 years 255 days  
Options Outstanding, Weighted-average Exercise Price Per Share (in dollars per share) $ 5.45  
Shares Subject to Stock Options (in shares) 1,880,190  
Options Vested and Exercisable, Weighted-average Exercise Price Per Share (in dollars per share) $ 5.36  
Exercise Price Range 4 [Member]    
Exercise Price Range, Lower Range Limit (in dollars per share) 8.18  
Exercise Price Range, Upper Range Limit (in dollars per share) $ 10.55  
Number of Stock Options Outstanding (in shares) 696,000  
Weighted-average Remaining Contractual LIfe (Year) 6 years 73 days  
Options Outstanding, Weighted-average Exercise Price Per Share (in dollars per share) $ 10.28  
Shares Subject to Stock Options (in shares) 662,284  
Options Vested and Exercisable, Weighted-average Exercise Price Per Share (in dollars per share) $ 10.28