Note 6 - Stockholders' Equity, Stock Plans and Stock-based Compensation Expense - Valuation Assumptions and Fair Value of Stock Option and ESPP Grants (Details) - $ / shares		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Payment Arrangement, Option [Member]
Expected term (in years) (Year)	[1]	3 years 237 days	3 years 255 days	3 years 255 days
Risk-free interest rate	[2]	1.64%	2.60%	1.73%
Volatility	[3]	54.00%	44.00%	40.00%
Dividend yield	[4]
Weighted average estimated fair value at grant date (in dollars per share)		$ 14.83	$ 18	$ 9.98
Stock Purchase Plan [Member]
Expected term (in years) (Year)	[1]	182 days	182 days	182 days
Risk-free interest rate	[2]	2.49%	2.34%	1.14%
Volatility	[3]	70.00%	61.00%	42.00%
Dividend yield	[4]
Weighted average estimated fair value at grant date (in dollars per share)		$ 9.60	$ 9.60	$ 8.21

[1] The expected term represents the period during which the Company's stock-based awards are expected to be outstanding. The estimated term is based on historical experience of similar awards, giving consideration to the contractual terms of the awards, vesting requirements, and expectation of future employee behavior, including post-vesting terminations. The expected term of groups of employees that have similar historical exercise patterns has been considered separately for valuation purposes.

[2] The risk-free interest rate is based on U.S. Treasury debt securities with maturities close to the expected term of the option or ESSP participation right as of the date of grant.

[3] Estimated volatility is based on historical volatility. The Company estimates volatility based on a 50-50 blend of the Company's historical volatility and the implied volatility of freely traded options of the Company's stock in the open market.

[4] The Company has not paid dividends since its inception.