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Note 9 - Stockholders' Equity - Options Exercisable, Vested or Expected to Vest (Details) - $ / shares
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Options exercisable at end of year (in shares) 6,523,093 5,446,586 4,356,665
Options exercisable at end of year, weighted average exercise price (in dollars per share) $ 14.83 $ 13.25 $ 11.66
Options vested or expected to vest (in shares) 14,449,017 12,854,571 11,234,529
Options vested or expected to vest, weighted average exercise price (in dollars per share) $ 18.11 $ 14.98 $ 14.32