Note 9 - Stockholders' Equity and Stock-based Compensation Expense - Activity Under the 2004 Equity Incentive Plan (Details)	9 Months Ended
	Sep. 30, 2018 $ / shares shares
Balances, shares available for grant (in shares)	1,494,865
Balances, stock options outstanding (in shares)	839,919
Balances, weighted-average exercise price (in dollars per share) | $ / shares	$ 16.46
Options granted, shares available for grant (in shares)	(21,010)
Options granted, stock options outstanding (in shares)	21,010
Options granted, weighted-average exercise price (in dollars per share) | $ / shares	$ 50.65
Stock awards granted, shares available for grant (in shares)	(530,876)	[1]
Stock awards granted, stock options outstanding (in shares)		[1]
Stock awards granted, weighted-average exercise price (in dollars per share) | $ / shares		[1]
Options exercised, stock options outstanding (in shares)	(241,021)
Options exercised, weighted-average exercise price (in dollars per share) | $ / shares	$ 10.16
Options cancelled (expired or forfeited), shares available for grant (in shares)	46,333
Options cancelled (expired or forfeited), weighted-average exercise price (in dollars per share) | $ / shares	$ 22.14
Stock awards cancelled (expired or forfeited), shares available for grant (in shares)	95,775	[1]
Balances, shares available for grant (in shares)	1,085,088
Balances, stock options outstanding (in shares)	573,575
Balances, weighted-average exercise price (in dollars per share) | $ / shares	$ 19.90

[1] The Company has a "fungible share" provision in its Amended and Restated 2004 Equity Incentive Plan whereby for each full-value award (RSU/PSU) issued or canceled under the Plan requires the subtraction or add back of 2.12 shares from or to the Shares Available for Grant, respectively.