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STOCK-BASED COMPENSATION (Tables) | 6 Months Ended
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Jun. 30, 2021
STOCK-BASED COMPENSATION |
Summary of restricted stock activity | ​
​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | Weighted
​ | ​ | Number of | ​ | Average Grant
​ |  | Shares |  | Date Fair Value
Nonvested stock at December 31, 2020 |  | 342,082 | ​ | $ | 108.40
Granted |  | 282,966 | ​ | ​ | 90.42
Vested |  | (156,273) | ​ | ​ | 106.84
Forfeited |  | (5,496) | ​ | ​ | 95.72
Nonvested stock at June 30, 2021 |  | 463,279 | ​ | $ | 98.09
Summary of performance-based restricted stock unit | ​
​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | Weighted
​ | ​ | Number of | ​ | Average Grant
​ |  | Shares |  | Date Fair Value
Nonvested stock at December 31, 2020 |  | 122,666 | ​ | $ | 115.18
Granted |  | 71,302 | ​ | ​ | 98.32
Vested |  | (29,468) | ​ | ​ | 111.45
Forfeited |  | (2,318) | ​ | ​ | 116.73
Nonvested stock at June 30, 2021 |  | 162,182 | ​ | $ | 108.42

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