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Note 9 - Stock-based Compensation (Tables) | 9 Months Ended
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Sep. 30, 2019
Notes Tables |
Share-based Payment Arrangement, Option, Activity [Table Text Block] |
|  |  |  |  |  | Weighted- |
|  |  |  |  |  | Average |
|  | Stock Options |  |  | Exercise Price |
Options outstanding at December 31, 2018 |  |  | 252,616 |  |  | $ | 64.71 |
Granted |  |  | 53,708 |  |  |  | 105.81 |
Exercised |  |  | (11,921 | ) |  |  | 56.25 |
Forfeited |  |  | (10,833 | ) |  |  | 83.74 |
Options outstanding at September 30, 2019 |  |  | 283,570 |  |  | $ | 72.12 |
Exercisable at September 30, 2019 |  |  | 154,539 |  |  | $ | 58.18 |
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block] |
|  | Nine Months Ended September 30, |
|  | 2019 |  |  | 2018 |
Risk-free interest rate |  | 2.35 - 2.58% |  |  | 2.52 - 3.07% |
Expected life (years) |  | 6.25 |  |  | 6.25 |
Expected volatility |  | 42.52 - 42.74% |  |  | 41.68 - 42.22% |
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, 2018 |  |  | 323,921 |  |  | $ | 79.85 |
Granted |  |  | 114,467 |  |  |  | 106.41 |
Restrictions lapsed |  |  | (95,884 | ) |  |  | 79.32 |
Forfeited |  |  | (20,299 | ) |  |  | 86.54 |
Restricted stock at September 30, 2019 |  |  | 322,205 |  |  | $ | 89.02 |
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, 2018 |  |  | 52,140 |  |  | $ | 76.54 |
Granted |  |  | 21,434 |  |  |  | 104.99 |
Restrictions lapsed |  |  | - |  |  |  | - |
Performance change |  |  | (12,854 | ) |  |  | 58.35 |
Forfeited |  |  | (2,375 | ) |  |  | 105.37 |
Performance stock at September 30, 2019 |  |  | 58,345 |  |  | $ | 89.82 |
Schedule of Share-based Payment Award, Employee Stock Purchase Plan, Valuation Assumptions [Table Text Block] |
|  | Nine Months Ended September 30, |
|  | 2019 |  |  | 2018 |
Risk-free interest rate |  | 2.33 - 2.35% |  |  | 1.48 - 2.06% |
Expected life (months) |  | 6.00 |  |  | 6.00 |
Expected volatility |  | 37.36 - 53.57% |  |  | 24.49 - 31.50% |
Expected dividend yield |  | 0% |  |  | 0% |

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- Definition Tabular disclosure for stock option plans. Includes, but is not limited to, outstanding awards at beginning and end of year, grants, exercises, forfeitures, and weighted-average grant date fair value. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 + Details
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- Definition Tabular disclosure of the significant assumptions used during the year to estimate the fair value of employee stock purchase plans, including, but not limited to: (a) expected term, (b) expected volatility of the entity's shares, (c) expected dividends, (d) risk-free rate(s), and (e) discount for post-vesting restrictions. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 + Details
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- Definition Tabular disclosure of the significant assumptions used during the year to estimate the fair value of stock options, including, but not limited to: (a) expected term of share options and similar instruments, (b) expected volatility of the entity's shares, (c) expected dividends, (d) risk-free rate(s), and (e) discount for post-vesting restrictions. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 + Details
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- Definition Disclosure of the number and weighted-average grant date fair value for restricted stock and restricted stock units that were outstanding at the beginning and end of the year, and the number of restricted stock and restricted stock units that were granted, vested, or forfeited during the year. + References No definition available. + Details
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