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Note 8 - Stockholders' Equity - Option Summary (Details) - $ / shares
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Number of shares outstanding (in shares) 8,434,988 7,918,004 6,355,681
Number of shares outstanding, weighted average exercise price (in dollars per share) $ 13.64 $ 8.69 $ 8.22
Number of shares granted (in shares) 4,920,661 2,134,505 3,181,001
Number of shares granted, weighted average exercise price (in dollars per share) $ 15.98 $ 28.95 $ 9.40
Number of shares exercised (in shares) (798,363) (1,042,343) (756,593)
Number of shares exercised, weighted average exercise price (in dollars per share) $ 8.37 $ 9.15 $ 6.34
Number of shares cancelled (in shares) (711,910) (575,178) (862,085)
Number of shares cancelled, weighted average exercise price (in dollars per share) $ 22.43 $ 10.42 $ 9.88
Number of shares outstanding (in shares) 11,845,376 8,434,988 7,918,004
Number of shares outstanding, weighted average exercise price (in dollars per share) $ 14.44 $ 13.64 $ 8.69