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COMMON UNIT-BASED COMPENSATION PLANS - LTIP Grants Activity (Details) - ARLP LTIP - USD ($) $ / shares in Units, $ in Thousands | 1 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2020 | Feb. 29, 2020 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Number of units |  |  |  |  |  |  |  |  |  |
Balance at the beginning of the period (in units) |  |  |  | 1,603,378 |  | 1,828,080 |  | 1,694,026 |  |
Granted (in units) |  |  |  | 1,430,489 | [1] | 682,155 |  | 511,305 |  | 424,486
Vested (in units) |  |  |  | (919,524) | [2] | (885,381) | [3] | (331,502) | [3] |
Grants canceled (in units) | [4] |  |  | (675,302) |  |  |  |  |  |
Forfeited (in units) |  |  |  | (8,552) |  | (21,476) |  | (45,749) |  |
Balance at the end of the period (in units) |  | 1,430,489 |  | 1,430,489 |  | 1,603,378 |  | 1,828,080 |  | 1,694,026
Weighted average grant date fair value per unit |  |  |  |  |  |  |  |  |  |
Balance at the beginning of the period (in dollars per unit) |  |  |  | $ 20.39 |  | $ 17.18 |  | $ 19.62 |  |
Granted (in dollars per unit) |  |  |  | 5.02 | [1] | 18.63 |  | 20.40 |  |
Vested (in dollars per unit) |  |  |  | $ 21.70 | [2] | 12.38 | [3] | 34.61 | [3] |
Grants canceled (in dollars per unit) | [4] |  |  | 18.62 |  |  |  |  |  |
Forfeited (in dollars per unit) |  |  |  | $ 20.16 |  | 20.84 |  | 17.40 |  |
Balance at the end of the period (in dollars per unit) |  | $ 5.02 |  | $ 5.02 |  | $ 20.39 |  | $ 17.18 |  | $ 19.62
Intrinsic value (in dollars) |  |  |  |  |  |  |  |  |  |
Intrinsic value of outstanding grants (in dollars) |  | $ 6,409 |  | $ 6,409 |  | $ 17,349 |  | $ 31,699 |  | $ 33,372
Other information |  |  |  |  |  |  |  |  |  |
Common units issued upon vesting |  |  | 279,622 |  |  | 596,650 |  | 191,858 |  |
2018 Grants |  |  |  |  |  |  |  |  |  |
Number of units |  |  |  |  |  |  |  |  |  |
Vested (in units) |  | (495,038) |  |  |  |  |  |  |  |
2019 Grants |  |  |  |  |  |  |  |  |  |
Number of units |  |  |  |  |  |  |  |  |  |
Grants canceled (in units) |  | 675,302 |  |  |  |  |  |  |  |
2020 Grants |  |  |  |  |  |  |  |  |  |
Number of units |  |  |  |  |  |  |  |  |  |
Granted (in units) |  | 578,114 |  |  |  |  |  |  |  |
[1] | 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.
[2] | 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.
[3] | 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.
[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.

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- Definition The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + 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 (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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