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	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Summary of non-vested grants (in units)
Balance at the beginning of the period (in units)	429,141	368,981	347,320
Granted (in units)	74,799	60,160	27,577
Issued (in units)	(9,922)		(5,916)
Balance at the end of the period (in units)	494,018	429,141	368,981
Weighted average grant date fair value per unit
Balance at the beginning of the period (in dollars per unit)	$ 32.25	$ 34.02	$ 33.20
Granted (in dollars per unit)	16.31	21.38	44.56
Issued (in dollars per unit)	33.76		35.29
Balance at the end of the period (in dollars per unit)	$ 29.77	$ 32.25	$ 34.02
Intrinisic value (in dollars)
Intrinsic value of outstanding grants (in dollars).	$ 11,091	$ 5,789	$ 15,885	$ 13,372