Note 8 - Stockholders' Equity and Stock-based Compensation Expense - Activity Under the 2004 Equity Incentive Plan (Details)	3 Months Ended
	Mar. 31, 2020 $ / shares shares
Balance, shares available for grant (in shares)	761,705
Balance, stock options outstanding (in shares)	295,699
Balance, weighted-average exercise price (in dollars per share) | $ / shares	$ 25.52
Stock awards granted, shares available for grant (in shares)	(434,797)
Options exercised, stock options outstanding (in shares)	(22,291)
Options exercised, weighted-average exercise price (in dollars per share) | $ / shares	$ 8.98
Options canceled, shares available for grant (in shares)	19,560
Options canceled, stock options outstanding (in shares)	(19,560)
Options canceled, weighted-average exercise price (in dollars per share) | $ / shares	$ 43.91
Stock awards canceled (in shares)	343,911
Balance, shares available for grant (in shares)	690,379
Balance, stock options outstanding (in shares)	253,848
Balance, weighted-average exercise price (in dollars per share) | $ / shares	$ 25.55