COMPENSATION PLANS - SERP and Directors Deferred Compensation Plans (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Feb. 08, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
SERP and Deferred Compensation Plans
Summary of non-vested grants (in units)
Issued (in units)	(7,181)
SERP And MGP Deferred Compensation Plan | Phantom Share Units (PSUs)
Summary of non-vested grants (in units)
Balance at the beginning of the period (in units)		494,018	429,141	368,981
Granted (in units)		67,766	74,799	60,160
Issued (in units)			(9,922)
Balance at the end of the period (in units)		561,784	494,018	429,141
Weighted average grant date fair value per unit
Balance at the beginning of the period (in dollars per unit)		$ 29.77	$ 32.25	$ 34.02
Granted (in dollars per unit)		20.38	16.31	21.38
Issued (in dollars per unit)			33.76
Balance at the end of the period (in dollars per unit)		$ 28.64	$ 29.77	$ 32.25
Intrinsic value (in dollars)
Intrinsic value of outstanding grants (in dollars)		$ 11,067	$ 11,091	$ 5,789	$ 15,885