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Note 6 - Stock-Based Compensation (Details Textual) $ / shares in Units, shares in Thousands | 1 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Aug. 31, 2013 shares | Dec. 31, 2015 USD ($) $ / shares shares | Dec. 31, 2014 USD ($) $ / shares shares | Dec. 31, 2013 USD ($) $ / shares shares | Dec. 31, 2012 shares
Incentive Plan [Member] | Employee Stock Option [Member] | Vest 25% Each Year Until Fully Vested [Member] |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage |  | 25.00% |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period |  | 4 years |  |  |
Incentive Plan [Member] | Employee Stock Option [Member] | Non-employee Directors [Member] |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, First Vesting Period After Grant Date |  | 1 year |  |  |
Incentive Plan [Member] | Employee Stock Option [Member] | Minimum [Member] |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term |  | 5 years |  |  |
Incentive Plan [Member] | Employee Stock Option [Member] | Maximum [Member] |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term |  | 10 years |  |  |
Incentive Plan [Member] | Stock Options and Restricted Stock [Member] | Vest 50% Each Year Until Fully Vested [Member] |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage |  | 50.00% |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period |  | 2 years |  |  |
Incentive Plan [Member] | Performance Shares [Member] | Vest Upon Successful Completion of Specific Development Milestones [Member] | August 2013 [Member] |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage |  | 75.00% |  |  |
Number of Milestones Achieved |  | 3 |  |  |
Share-based Compensation Arrangement by Shar-based Payment Award, Awards Unvested, Percentage |  | 25.00% |  |  |
Incentive Plan [Member] | Performance Shares [Member] | Vest Upon Successful Completion of Specific Development Milestones [Member] | December 2014 [Member] |  |  |  |  |
Share-based Compensation Arrangement by Shar-based Payment Award, Awards Unvested, Percentage |  | 100.00% |  |  |
Incentive Plan [Member] | Performance Shares [Member] | Vest Upon Successful Completion of Specific Development Milestones [Member] |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross | shares | 1,032 | 1,250 |  |  |
Incentive Plan [Member] |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value |  | $ 0 |  |  |
Allocated Share-based Compensation Expense |  | $ 9,485,000 | $ 9,963,000 | $ 4,253,000 |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term |  | 5 years |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value | $ / shares |  | $ 7.72 | $ 8.02 | $ 1.28 |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value |  | $ 10,117,000 | $ 8,522,000 | $ 738,000 |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value |  | 28,511,000 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested in Period, Fair Value |  | $ 4,492,000 | 2,844,000 | 3,483,000 |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Number | shares |  | 9,633 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Weighted Average Exercise Price | $ / shares |  | $ 7.51 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Weighted Average Remaining Contractual Term |  | 6 years 219 days |  |  |
Employee Stock Purchase Plan [Member] |  |  |  |  |
Allocated Share-based Compensation Expense |  | $ 220,000 | $ 214,000 | $ 115,000 |
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized | shares |  | 1,475 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant | shares |  | 497 |  |  |
Percentage of Salary to Purchase Common Stock, Maximum |  | 15.00% |  |  |
Percentage of Common Stock Shares, Beginning |  | 85.00% |  |  |
Percentage of Common Stock Shares, Ending |  | 85.00% |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Maximum Number of Shares Per Employee | shares |  | 3 |  |  |
Shar-based Compensation Arrangement by Shar-based Payment Award, Maximum Number of Shares Per Employee, Amount |  | $ 25,000 |  |  |
Employee stock purchase plan sales, shares (in shares) | shares |  | 41 | 49 | 89 |
Employee Stock Ownership Plan (ESOP), Weighted Average Purchase Price of Shares Purchased | $ / shares |  | $ 8.65 | $ 6.29 | $ 1.39 |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value | $ / shares |  | $ 4.93 | $ 4.41 | $ 1.27 |
Number of Stock-based Compensation Plans |  | 2 |  |  |
Allocated Share-based Compensation Expense |  | $ 9,705,000 | $ 10,177,000 | $ 4,368,000 |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross | shares |  | 2,217 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value | $ / shares |  | $ 7.72 |  |  |
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized |  | $ 18,607,000 |  |  |
Employee Service Share-based Compensation Nonvested Awards Compensation Cost Expected to be Recognized For Remainder of Fiscal Year |  | 7,023,000 |  |  |
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Expected to be Recognized Year Two |  | 6,010,000 |  |  |
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Expected to be Recognized Year Three |  | 3,891,000 |  |  |
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Expected to be Recognized Year Four |  | $ 1,683,000 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant | shares |  | 16 | 2,362 | 1,082 | 2,815

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