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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.