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STOCK-BASED COMPENSATION (Tables) | 3 Months Ended
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Mar. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
Schedule of stock-based compensation | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended March 31, | ​
​ |  | 2020 |  | 2019 | ​
Stock-based compensation expense | ​ | $ | 3,048 | ​ | $ | 3,199 | ​
Summary of stock option activity | Changes in our outstanding stock options during the three months ended March 31, 2020 were as follows:
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended March 31, 2020 | ​
​ |  | ​ |  | Weighted- | ​
​ | ​ | ​ | ​ | Average | ​
​ | ​ | Number of | ​ | Exercise Price | ​
​ | ​ | Options | ​ | per Share | ​
Options outstanding at beginning of period |  | 185 | ​ | $ | 21.56 |
Options exercised |  | (5) | ​ | $ | 15.73 |
Options forfeited |  | — | ​ | $ | — |
Options expired |  | (2) | ​ | $ | 15.65 |
Options outstanding at end of period |  | 178 | ​ | $ | 21.79 |
Options vested during the year |  | — | ​ |  |  |
Restricted Stock Units (RSUs) [Member] |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
Summary of stock option activity | Changes in the outstanding RSU awards during the three months ended March 31, 2020 were as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended March 31, 2020 | ​
​ |  | ​ |  | Weighted- | ​
​ | ​ | ​ | ​ | Average | ​
​ | ​ | ​ | ​ | Grant Date | ​
​ | ​ | Number of RSUs | ​ | Fair Value | ​
RSUs outstanding at beginning of period |  | 534 | ​ | $ | 56.56 |
RSUs granted |  | 293 | ​ | $ | 59.32 |
RSUs vested |  | (94) | ​ | $ | 58.47 |
RSUs forfeited |  | (13) | ​ | $ | 61.29 |
RSUs outstanding at end of period |  | 720 | ​ | $ | 57.34 |

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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 Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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