Note 8 - Stock-based Compensation (Tables)
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9 Months Ended |
Sep. 30, 2016 |
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| Notes Tables |
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| Schedule of Share-based Compensation, Stock Options, Activity [Table Text Block] |
| | | | | | | | | Options outstanding at December 31, 2015 | | | 766,042 | | | $ | 36.52 | | | Granted | | | 113,645 | | | | 58.12 | | | Exercised | | | (173,781 | ) | | | 18.42 | | | Forfeited | | | (69,965 | ) | | | 61.37 | | | Options outstanding at September 30, 2016 | | | 635,941 | | | $ | 42.59 | | | | | | | | | | | | | Exercisable at September 30, 2016 | | | 368,052 | | | $ | 31.13 | | |
| Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block] |
| | | Nine Months Ended September 30, | | | | | | | | | | | Risk-free interest rate | | 1.53 | - | 1.68% | | | 1.69 | - | 1.77% | | | Expected life (years) | | | 6.50 | | | | 5.50 | - | 6.50 | | | Expected volatility | | 44.38 | - | 45.93% | | | 46.80 | - | 47.23% | | | Expected dividend yield | | | 0% | | | | | 0% | | | |
| Schedule of Share-based Compensation, Restricted Stock and Restricted Stock Units Activity [Table Text Block] |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Restricted stock at December 31, 2015 | | | 124,393 | | | $ | 68.97 | | | Granted | | | 159,705 | | | | 59.43 | | | Restrictions lapsed | | | (40,779 | ) | | | 67.85 | | | Forfeited | | | (19,645 | ) | | | 63.99 | | | Restricted stock at September 30, 2016 | | | 223,674 | | | $ | 62.80 | | |
| Schedule of Share-based Payment Award, Employee Stock Purchase Plan, Valuation Assumptions [Table Text Block] |
| | | Nine Months Ended September 30, | | | | | | | | | | | Risk-free interest rate | | 0.39 | - | 0.56% | | | 0.08 | - | 0.10% | | | Expected life (months) | | | 6.00 | | | | | 6.00 | | | | Expected volatility | | 29.41 | - | 49.13% | | | 33.68 | - | 37.64% | | | Expected dividend yield | | | 0% | | | | | 0% | | | |