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COMPENSATION PLANS (Tables) | 12 Months Ended
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Dec. 31, 2019
ARLP LTIP |
Compensation Plans |
Summary of activity in share-based plans | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | Number of units |  | Weighted average grant date fair value per unit |  | Intrinsic value |
​ | ​ | ​ | ​ | ​ | ​ | ​ | (in thousands) | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Non-vested grants at January 1, 2017 | ​ | 1,604,748 | ​ | $ | 23.19 | ​ | $ | 36,027 | ​
Granted | ​ | 475,310 | ​ | ​ | 23.17 | ​ | ​ | ​ | ​
Vested (1) | ​ | (350,516) | ​ | ​ | 40.73 | ​ | ​ | ​ | ​
Forfeited | ​ | (35,516) | ​ | ​ | 20.01 | ​ | ​ | ​ | ​
Non-vested grants at December 31, 2017 | ​ | 1,694,026 | ​ | ​ | 19.62 | ​ | ​ | 33,372 | ​
Granted | ​ | 511,305 | ​ | ​ | 20.40 | ​ | ​ | ​ | ​
Vested (1) | ​ | (331,502) | ​ |  | 34.61 | ​ | ​ | ​ | ​
Forfeited | ​ | (45,749) | ​ |  | 17.40 | ​ | ​ | ​ | ​
Non-vested grants at December 31, 2018 | ​ | 1,828,080 | ​ | ​ | 17.18 | ​ | ​ | 31,699 | ​
Granted |  | 682,155 | ​ | ​ | 18.63 | ​ | ​ | ​ | ​
Vested (1) |  | (885,381) | ​ |  | 12.38 | ​ | ​ | ​ | ​
Forfeited |  | (21,476) | ​ |  | 20.84 | ​ | ​ | ​ | ​
Non-vested grants at December 31, 2019 |  | 1,603,378 | ​ |  | 20.39 | ​ | ​ | 17,349 | ​
(1) | During the years ended December 31, 2019, 2018 and 2017, we issued 596,650, 191,858 and 222,011, respectively, unrestricted common units to the LTIP participants. The remaining vested units were settled in cash primarily to satisfy tax withholding obligations of the LTIP participants.
SERP and Directors' Compensation Plans |
Compensation Plans |
Summary of activity in share-based plans |
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | Number of units |  | Weighted average grant date fair value per unit |  | Intrinsic value |
​ | ​ | ​ | ​ | ​ | ​ | ​ | (in thousands) | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Phantom units outstanding as of January 1, 2017 | ​ | 494,018 | ​ | $ | 29.77 | ​ | $ | 11,091 | ​
Granted | ​ | 67,766 | ​ | ​ | 20.38 | ​ | ​ | ​ | ​
Phantom units outstanding as of December 31, 2017 | ​ | 561,784 | ​ | ​ | 28.64 | ​ | ​ | 11,067 | ​
Granted | ​ | 84,417 | ​ | ​ | 18.78 | ​ | ​ | ​ | ​
Issued (1) | ​ | (10,364) | ​ | ​ | 27.92 | ​ | ​ | ​ | ​
Phantom units outstanding as of December 31, 2018 | ​ | 635,837 | ​ | ​ | 27.34 | ​ | ​ | 11,025 | ​
Granted | ​ | 111,012 | ​ | ​ | 14.50 | ​ | ​ | ​ | ​
Issued (1) |  | (115,484) | ​ | ​ | 25.20 | ​ | ​ | ​ | ​
Phantom units outstanding as of December 31, 2019 |  | 631,365 | ​ |  | 25.48 | ​ | ​ | 6,831 | ​
(1) | During the years ended December 31, 2019 and 2018, we issued ARLP common units of 115,484 and 7,181, respectively, to participants under the SERP and Directors' Deferred Compensation Plan. Units issued in 2018 were net of units settled in cash to satisfy tax withholding obligations.

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