COMPENSATION PLANS - SERP and Directors Deferred Compensation Plans (Details) - SERP And MGP Deferred Compensation Plan - Phantom Share Units (PSUs) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Summary of non-vested grants (in units)
Balance at the beginning of the period (in units)	494,018
Granted (in units)	12,307
Balance at the end of the period (in units)	506,325
Weighted average grant date fair value per unit
Balance at the beginning of the period (in dollars per unit)	$ 29.77
Granted (in dollars per unit)	23.22
Balance at the end of the period (in dollars per unit)	$ 29.61
Intrinisic value (in dollars)
Intrinsic value of outstanding grants (in dollars).	$ 10,962	$ 11,091