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COMMON UNIT-BASED COMPENSATION PLANS (Tables) | 12 Months Ended
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Dec. 31, 2020
ARLP LTIP |
COMMON UNIT-BASED 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, 2018 | ​ | 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 | ​
Granted (2) |  | 1,430,489 | ​ | ​ | 5.02 | ​ | ​ | ​ | ​
Vested (3) |  | (919,524) | ​ |  | 21.70 | ​ | ​ | ​ | ​
Grants canceled (4) | ​ | (675,302) | ​ | ​ | 18.62 | ​ | ​ | ​ | ​
Forfeited |  | (8,552) | ​ |  | 20.16 | ​ | ​ | ​ | ​
Non-vested grants at December 31, 2020 |  | 1,430,489 | ​ |  | 5.02 | ​ | ​ | 6,409 | ​
(1) | During the years ended December 31, 2019 and 2018, we issued 596,650 and 191,858, respectively, unrestricted common units to LTIP participants. The remaining vested units were settled in cash to satisfy tax withholding obligations of the LTIP participants.
(2) | In December 2020, we modified the vesting requirements for certain restricted units that we granted in February 2020 which were determined to be improbable of vesting under the original vesting requirements (the "2020 Grants"). The new vesting requirements make it probable the modified restricted units will vest. Also in December 2020, an additional 578,114 restricted units under these modified vesting requirements were granted. The grant date fair value reflects the modification date fair value for those awards that were modified.
(3) | In February 2020, we issued 279,622 unrestricted common units to LTIP participants as a result of satisfying the vesting requirements for 424,486 restricted units that were granted in 2017. The remaining vested units were settled in cash to satisfy tax withholding obligations of the LTIP participants. In December 2020, we accelerated the vesting requirements for 495,038 restricted units that were granted in 2018 (the "2018 Grants") and settled these restricted units in cash.
(4) | In December 2020, 675,302 restricted units that were granted in 2019 (the "2019 Grants") were canceled since it was determined that the vesting requirements for these restricted units were not probable of being satisfied.
SERP and Directors' Compensation Plans |
COMMON UNIT-BASED 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, 2018 | ​ | 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 | ​
Granted | ​ | 129,265 | ​ | ​ | 5.25 | ​ | ​ | ​ | ​
Phantom units outstanding as of December 31, 2020 |  | 760,630 | ​ |  | 22.04 | ​ | ​ | 3,408 | ​
(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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