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STOCK-BASED COMPENSATION (Tables) | 12 Months Ended
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Dec. 31, 2020
Schedule of stock-based compensation | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Years Ended December 31,
​ |  | 2020 |  | 2019 |  | 2018
Stock-based compensation expense | ​ | $ | 12,272 | ​ | $ | 7,327 | ​ | $ | 9,703
Disclosure of compensation related costs | ​
​ | ​ | ​ | ​ | ​ | ​
​ | ​ | 2020
​ | ​ | ​ |  | Weighted-
​ | ​ | ​ | ​ | Average
​ | ​ | ​ | ​ | Grant
​ |  | Shares | ​ | Value
RSUs outstanding at beginning of period |  | 534 | ​ | $ | 56.56
RSUs granted |  | 342 | ​ |  | 59.51
RSUs vested |  | (152) | ​ |  | 57.14
RSUs forfeited |  | (116) | ​ |  | 58.10
RSUs outstanding at end of period |  | 608 | ​ | $ | 58.15
Schedule of shares authorized under stock option plans, by exercise price range | The following table summarizes information about the stock options outstanding on December 31, 2020: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Options Outstanding and Exercisable
​ | ​ | ​ | ​ | Weighted-Average | ​ | Weighted-
​ | ​ | Number | ​ | Remaining | ​ | Average
Range of Exercise Prices |  | Outstanding |  | Contractual Life |  | Exercise Price
$9.51 to $12.44 |  | 5 |  | 0.67 years | ​ | $ | 11.18
$18.77 |  | 43 |  | 3.75 years | ​ |  | 18.77
$26.32 |  | 99 |  | 4.10 years | ​ |  | 26.32
$9.51 to $26.32 |  | 147 |  | 3.89 years | ​ | $ | 23.63
Employee Stock [Member] |
Schedule of valuation assumptions | The fair value of each purchase right granted under the ESPP was estimated on the date of grant using the Black-Scholes-Merton option pricing model with the following assumptions: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Years Ended December 31, | ​
​ |  | 2020 |  | 2019 |  | 2018 |
Risk-free interest rates |  | 0.10% - 0.18% | % | 1.62% - 2.31 | % | 2.10% - 2.56 | %
Expected dividend yield rates |  | — | % | — | % | — | %
Expected term |  | 0.5 years |  | 0.5 years |  | 0.5 years | ​
Expected volatility |  | 70.1 | % | 41.3 | % | 38.0 | %
Time Based [Member] |
Schedule of stock options roll forward | Changes in our outstanding stock options were as follows: ​
​ | ​ | ​ | ​ | ​ | ​
​ | ​ | 2020
​ | ​ | ​ |  | Weighted-
​ | ​ | ​ | ​ | Average
​ | ​ | ​ | ​ | Exercise
​ |  | Shares | ​ | Price
Options outstanding at beginning of period |  | 185 | ​ | $ | 21.56
Options exercised |  | (33) | ​ |  | 13.43
Options forfeited |  | — | ​ |  | —
Options expired |  | (5) | ​ |  | 15.43
Options outstanding at end of period |  | 147 | ​ | $ | 23.63
Schedule of restricted stock and restricted stock units activity | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Options Outstanding and Exercisable
​ | ​ | ​ | ​ | Weighted-Average | ​ | Weighted-
​ | ​ | Number | ​ | Remaining | ​ | Average
Range of Exercise Prices |  | Outstanding |  | Contractual Life |  | Exercise Price
$9.51 to $12.44 |  | 5 |  | 0.67 years | ​ | $ | 11.18
$18.77 |  | 43 |  | 3.75 years | ​ |  | 18.77
$26.32 |  | 99 |  | 4.10 years | ​ |  | 26.32
$9.51 to $26.32 |  | 147 |  | 3.89 years | ​ | $ | 23.63

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