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COMPENSATION PLANS - SERP and Directors Compensation Activity (Details) - SERP and Directors' Compensation Plans - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Other information        
Unit-based compensation expense $ 1,600 $ 1,600 $ 1,400  
Total unit-based obligation recorded $ 16,100 $ 17,400    
Phantom Share Units (PSUs)        
Summary of units        
Balance at the beginning of the period (in units) 635,837 561,784 494,018  
Granted (in units) 111,012 84,417 67,766  
Issued (in units) (115,484) (10,364)    
Balance at the end of the period (in units) 631,365 635,837 561,784  
Weighted average grant date fair value per unit        
Balance at the beginning of the period (in dollars per unit) $ 27.34 $ 28.64 $ 29.77  
Granted (in dollars per unit) 14.50 18.78 20.38  
Issued (in dollars per unit) 25.20 27.92    
Balance at the end of the period (in dollars per unit) $ 25.48 $ 27.34 $ 28.64  
Intrinsic value (in dollars)        
Intrinsic value of outstanding grants (in dollars) $ 6,831 $ 11,025 $ 11,067 $ 11,091
Other information        
Common units issued upon vesting 115,484 7,181