COMPENSATION PLANS - AGP Deferred Compensation Plan (Details) - AGP Deferred Compensation Plan - Phantom Share Units (PSUs) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Summary of non-vested grants (in units)
Balance at the beginning of the period (in units)	26,112	21,687	19,376
Granted (in units)	2,168	11,383	5,478
Issued (in units)	(4,357)	(6,958)	(3,167)
Balance at the end of the period (in units)	23,923	26,112	21,687
Weighted average grant date fair value per unit
Balance at the beginning of the period (in dollars per unit)	$ 36.67	$ 50.38	$ 53.96
Granted (in dollars per unit)	27.28	19.15	40.87
Issued (in dollars per unit)	39.18	50.73	55.83
Balance at the end of the period (in dollars per unit)	$ 35.36	$ 36.67	$ 50.38
Intrinsic value (in dollars)
Intrinsic value of outstanding grants (in dollars)	$ 641	$ 734	$ 438	$ 1,182