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Note 9 - Stock-based Compensation (Tables) | 9 Months Ended
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Sep. 30, 2021
Notes Tables |
Share-based Payment Arrangement, Option, Activity [Table Text Block] |
|  |  |  |  |  | Weighted- |
|  |  |  |  |  | Average |
|  | Stock Options |  |  | Exercise Price |
Options outstanding at December 31, 2020 |  |  | 229,531 |  |  | $ | 86.46 |
Granted |  |  | 57,901 |  |  |  | 128.14 |
Exercised |  |  | (28,991 | ) |  |  | 58.78 |
Forfeited |  |  | (9,893 | ) |  |  | 100.58 |
Options outstanding at September 30, 2021 |  |  | 248,548 |  |  | $ | 98.84 |
Exercisable at September 30, 2021 |  |  | 116,088 |  |  | $ | 81.47 |
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block] |
|  | Nine Months Ended September 30, |
|  | 2021 |  |  | 2020 |
Risk-free interest rate |  |  | 0.80 - 1.12% |  |  |  | 0.50 - 1.47% |
Expected life (years) |  |  | 6.25 |  |  |  | 6.25 |
Expected volatility |  |  | 45.28 - 45.53% |  |  |  | 42.40 - 43.83% |
Expected dividend yield |  |  | 0% |  |  |  | 0% |
Share-based Payment Arrangement, Restricted Stock and Restricted Stock Unit, Activity [Table Text Block] |
|  |  |  |  |  | Weighted- |
|  |  |  |  |  | Average |
|  |  |  |  |  | Grant Date |
|  | Restricted |  |  | Fair Value |
|  | Stock |  |  | Per Share |
Restricted stock at December 31, 2020 |  |  | 307,199 |  |  | $ | 98.87 |
Granted |  |  | 180,798 |  |  |  | 132.84 |
Restrictions lapsed |  |  | (113,315 | ) |  |  | 91.10 |
Forfeited |  |  | (22,908 | ) |  |  | 120.01 |
Restricted stock at September 30, 2021 |  |  | 351,774 |  |  | $ | 117.51 |
Schedule of Nonvested Performance-based Units Activity [Table Text Block] |
|  |  |  |  |  | Weighted- |
|  |  |  |  |  | Average |
|  |  |  |  |  | Grant Date |
|  | Performance |  |  | Fair Value |
|  | Stock |  |  | Per Share |
Performance stock at December 31, 2020 |  |  | 19,956 |  |  | $ | 118.66 |
Granted |  |  | 15,078 |  |  |  | 203.64 |
Restrictions lapsed |  |  | - |  |  |  | - |
Performance change |  |  | (8,226 | ) |  |  | 192.95 |
Forfeited |  |  | (4,184 | ) |  |  | 99.59 |
Performance stock at September 30, 2021 |  |  | 22,624 |  |  | $ | 148.28 |
Schedule of Share-based Payment Award, Awards Other Than Stock Options, Valuation Assumptions [Table Text Block] |
|  | Nine Months Ended September 30, |
|  | 2021 |  |  | 2020 |
Risk-free interest rate |  |  | 0.22% |  |  |  | 1.41% |
Expected life (years) |  |  | 2.87 |  |  |  | 2.88 |
Expected volatility |  |  | 51.40% |  |  |  | 38.70% |
Expected dividend yield |  |  | 0% |  |  |  | 0% |
Schedule of Share-based Payment Award, Employee Stock Purchase Plan, Valuation Assumptions [Table Text Block] |
|  | Nine Months Ended September 30, |
|  | 2021 |  |  | 2020 |
Risk-free interest rate |  |  | 0.06 - 0.12% |  |  |  | 0.17 - 1.59% |
Expected life (months) |  |  | 6.00 |  |  |  | 6.00 |
Expected volatility |  |  | 50.85 - 65.53% |  |  |  | 42.63 - 59.99% |
Expected dividend yield |  |  | 0% |  |  |  | 0% |

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