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, 2017	Dec. 31, 2016	Dec. 31, 2015
Employee Stock Option [Member]
Expected term (in years) (Year)	[1]	3 years 255 days	3 years 302 days	3 years 87 days
Risk-free interest rate	[2]	1.73%	1.09%	0.90%
Volatility	[3]	40.00%	40.00%	30.00%
Dividend yield	[4]
Weighted average estimated fair value at grant date (in dollars per share)		$ 9.98	$ 3.72	$ 4.78
Stock Purchase Plan [Member]
Expected term (in years) (Year)	[1]	182 days	182 days	182 days
Risk-free interest rate	[2]	1.14%	0.46%	0.17%
Volatility	[3]	42.00%	39.00%	36.00%
Dividend yield	[4]
Weighted average estimated fair value at grant date (in dollars per share)		$ 8.21	$ 3.22	$ 3.51

[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 as of the date of grant.

[3] Estimated volatility is based on historical volatility. The Company also considers implied volatility when there is sufficient volume of freely traded options with comparable terms and exercise prices in the open market.

[4] The Company has not historically issued any dividends and does not expect to do so in the foreseeable future.