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COMMON UNIT-BASED COMPENSATION PLANS - SERP and Directors Compensation Activity (Details) - SERP and Directors' Compensation Plans - USD ($) $ / shares in Units, $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Other information |  |  |  |  |
Unit-based compensation expense |  | $ 700 | $ 1,600 | $ 1,600 |
Total unit-based obligation recorded |  | $ 16,800 | $ 16,100 |  |
Phantom Share Units (PSUs) |  |  |  |  |
Number of units |  |  |  |  |
Balance at the beginning of the period (in units) |  | 631,365 | 635,837 | 561,784 |
Granted (in units) |  | 129,265 | 111,012 | 84,417 |
Issued (in units) | [1] |  | (115,484) | (10,364) |
Balance at the end of the period (in units) |  | 760,630 | 631,365 | 635,837 |
Weighted average grant date fair value per unit |  |  |  |  |
Balance at the beginning of the period (in dollars per unit) |  | $ 25.48 | $ 27.34 | $ 28.64 |
Granted (in dollars per unit) |  | 5.25 | 14.50 | 18.78 |
Issued (in dollars per unit) | [1] |  | 25.20 | 27.92 |
Balance at the end of the period (in dollars per unit) |  | $ 22.04 | $ 25.48 | $ 27.34 |
Intrinsic value (in dollars) |  |  |  |  |
Intrinsic value of outstanding grants (in dollars) |  | $ 3,408 | $ 6,831 | $ 11,025 | $ 11,067
Other information |  |  |  |  |
Common units issued upon vesting |  |  | 115,484 | 7,181 |
[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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