Note 8 - Stockholders' Equity (Details Textual) $ / shares in Units, $ in Thousands			12 Months Ended				48 Months Ended
	Jun. 30, 2015 shares	Nov. 14, 2013 shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2013 shares	Dec. 31, 2017 USD ($) $ / shares shares	Jan. 01, 2018 shares	Jan. 01, 2017 shares	Jan. 01, 2016 shares
Common Stock, Shares Authorized			200,000,000	200,000,000			200,000,000
Common Stock, Par or Stated Value Per Share | $ / shares			$ 0.001	$ 0.001			$ 0.001
Common Stock, Number of Voting Rights			1
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross			0	135,000	191,200
Common Stock, Capital Shares Reserved for Future Issuance, Annual Increase, Maximum									1,072,870	1,042,766
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value | $ / shares			$ 0	$ 16.87	$ 28.637
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value | $			$ 22,382	$ 9,418	$ 27,885
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized | $			$ 2,208	$ 3,380			$ 2,208
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition			323 days	1 year 282 days
Subsequent Event [Member]
Common Stock, Capital Shares Reserved for Future Issuance, Annual Increase, Maximum								1,125,846
The 2005 Stock Plan [Member]
Common Stock, Capital Shares Reserved for Future Issuance						4,713,319
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period						4 years
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross							0
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant			0				0
The 2013 Omnibus Equity Award Plan [Member]
Common Stock, Capital Shares Reserved for Future Issuance		1,904,633
Share-based Compensation Arrangement by Share-based Payment Award, Percentage of Outstanding Stock Reserved for Grant		4.00%
Share-based Compensation Arrangement by Share-based Payment Award, Percentage of Outstanding Stock Maximum		5.00%
The 2013 Omnibus Equity Award Plan [Member] | Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period		4 years
2015 ESPP [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Maximum Employee Subscription Rate	15.00%
Share-based Compensation Arrangement by Share-based Payment Award, Purchase Price of Common Stock, Percent	85.00%
Percent of Shares Increase, Employee Stock Purchase Plan	1.00%
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	500,000
Common Stock Availability Threshold, Employee Stock Purchase Plan	2.00%
Shares Increase Threshold, Employee Stock Purchase Plan	400,000
Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized			188,813	158,695	21,383