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Note 8 - Stockholders' Equity - Options Exercisable, Vested or Expected to Vest (Details) - $ / shares
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Options exercisable at end of year (in shares) 5,446,586 4,356,665 2,587,392
Options exercisable at end of year, weighted average exercise price (in dollars per share) $ 13.25 $ 11.66 $ 8.72
Options vested or expected to vest (in shares) 12,854,571 11,234,529 8,059,481
Options vested or expected to vest, weighted average exercise price (in dollars per share) $ 14.98 $ 14.32 $ 13.38