COMPENSATION PLANS - SERP and Directors Compensation Activity (Details) - SERP and Directors' Compensation Plans - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Other information
Unit-based compensation expense	$ 300	$ 400
Total unit-based obligation recorded	$ 16,300
Phantom Share Units (PSUs)
Summary of units
Balance at the beginning of the period (in units)	631,365
Granted (in units)	29,544
Balance at the end of the period (in units)	660,909
Weighted average grant date fair value per unit
Balance at the beginning of the period (in dollars per unit)	$ 25.48
Granted (in dollars per unit)	7.87
Balance at the end of the period (in dollars per unit)	$ 24.69
Intrinsic value (in dollars)
Intrinsic value of outstanding grants (in dollars)	$ 2,049		$ 6,831