Note 9 - Stockholders' Equity - Stock Option Activity (Details) - $ / shares	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Number of shares outstanding (in shares)	13,462,963	11,845,376	8,434,988
Number of shares outstanding, weighted average exercise price (in dollars per share)	$ 15.03	$ 14.44	$ 13.64
Number of shares granted (in shares)	4,052,011	4,300,621	4,920,661
Number of shares granted, weighted average exercise price (in dollars per share)	$ 26.83	$ 16.17	$ 15.98
Number of shares exercised (in shares)	(1,529,509)	(1,350,605)	(798,363)
Number of shares exercised, weighted average exercise price (in dollars per share)	$ 11.97	$ 8.49	$ 8.37
Number of shares cancelled (in shares)	(720,859)	(1,332,429)	(711,910)
Number of shares cancelled, weighted average exercise price (in dollars per share)	$ 17.98	$ 20.06	$ 22.43
Number of shares outstanding (in shares)	15,264,606	13,462,963	11,845,376
Number of shares outstanding, weighted average exercise price (in dollars per share)	$ 18.33	$ 15.03	$ 14.44

For presentations that combine terminations, the number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan or that expired.

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Weighted average price of options that were either forfeited or expired.

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Number of options outstanding, including both vested and non-vested options.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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