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STOCK-BASED COMPENSATION (Tables) | 6 Months Ended
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Jun. 30, 2021
STOCK-BASED COMPENSATION |
Schedule of stock-based compensation | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended June 30, | ​ | Six Months Ended June 30,
​ |  | 2021 |  | 2020 |  | 2021 |  | 2020
Stock-based compensation expense | ​ | $ | 3,444 | ​ | $ | 2,837 | ​ | $ | 9,145 | ​ | $ | 5,885
Summary of changes in RSUs | ​
​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Six Months Ended June 30, 2021
​ |  | ​ |  | Weighted-
​ | ​ | ​ | ​ | Average
​ | ​ | Number of | ​ | Grant Date
​ | ​ | RSUs | ​ | Fair Value
RSUs outstanding at beginning of period |  | 608 | ​ | $ | 58.15
RSUs granted |  | 358 | ​ | $ | 95.79
RSUs vested |  | (173) | ​ | $ | 62.80
RSUs forfeited |  | (145) | ​ | $ | 66.23
RSUs outstanding at end of period |  | 648 | ​ | $ | 75.93
Summary of changes in stock options | ​
​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Six Months Ended June 30, 2021
​ |  | ​ |  | Weighted-
​ | ​ | ​ | ​ | Average
​ | ​ | Number of | ​ | Exercise Price
​ | ​ | Options | ​ | per Share
Options outstanding at beginning of period |  | 147 | ​ | $ | 23.63
Options exercised |  | (23) | ​ | $ | 21.42
Options expired |  | — | ​ | $ | —
Options outstanding at end of period |  | 124 | ​ | $ | 24.03

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- Definition Tabular disclosure of activity for award under share-based payment arrangement. Includes, but is not limited to, outstanding award at beginning and end of year, granted, exercised, forfeited, 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 (e) -URI http://asc.fasb.org/extlink&oid=120381028&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 (c)(2) -URI http://asc.fasb.org/extlink&oid=120381028&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 (d) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Tabular disclosure of number, weighted-average exercise price or conversion ratio, aggregate intrinsic value, and weighted-average remaining contractual term for outstanding restricted stock units that are fully vested and expected to vest. Includes, but is not limited to, unvested restricted stock units for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur. + 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 (e)(1) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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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=120381028&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=120381028&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=120381028&loc=d3e5070-113901 + Details
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