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STOCK-BASED COMPENSATION (Tables) | 9 Months Ended
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Sep. 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 |  | 296,941 | ​ | ​ | 92.17
Vested |  | (161,305) | ​ | ​ | 106.44
Forfeited |  | (10,042) | ​ | ​ | 95.85
Nonvested stock at September 30, 2021 |  | 467,676 | ​ | $ | 99.04
Summary of performance-based restricted stock units | ​
​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | 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 September 30, 2021 |  | 162,182 | ​ | $ | 108.42

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