Note 9 - Stockholders' Equity and Stock-based Compensation Expense (Details Textual) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
Jun. 30, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period	188,859
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Total	$ 19.8
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition	2 years 182 days
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross	21,010
Non-employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross		7,745
Non-employees [Member] \| Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period	3,384	2,478
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	4 years
Non-employees [Member] \| Restricted Stock Units (RSUs) [Member] \| Vesting Each Anniversary of Grant Date [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage	25.00%
Non-employees [Member] \| Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period	3,384
Non-employees [Member] \| Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period		4 years
Non-employees [Member] \| Employee Stock Option [Member] \| Vesting on First Anniversary of Grant Date [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage		25.00%
Non-employees [Member] \| Employee Stock Option [Member] \| Vesting Each Month After First Anniversary of Grant Date [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage		0.02083%
1998 Stock Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	4,650,000
Two Thousand Four Equity Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	1,750,000
Number of Stock Awards Counted for Every Share Granted or Canceled	2.12
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period	346,279

The number of stack awards counted for every share granted or canceled when computed shares available for grant.

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Unrecognized cost of unvested share-based compensation awards.

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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

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Gross number of share options (or share units) granted during the period.

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Number of share options (or share units) exercised during the current period.

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