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COMPENSATION PLANS - LTIP Grants Activity (Details) - ARLP LTIP
9 Months Ended
Sep. 30, 2020
$ / shares
shares
Number of units  
Balance at the beginning of the period (in units) 1,603,378
Granted (in units) 950,035
Vested (in units) (424,486)
Forfeited (in units) (13,835)
Balance at the end of the period (in units) 2,115,092
Weighted average grant date fair value per unit  
Granted (in dollars per unit) | $ / shares $ 9.62
Vested (in dollars per unit) | $ / shares 23.22
Forfeited (in dollars per unit) | $ / shares $ 16.13
Other information  
Common units issued upon vesting 279,622
Units not expected to vest (in units) 675,302