COMPENSATION PLANS - AGP Deferred Compensation Plan (Details) - AGP Deferred Compensation Plan - Phantom Share Units (PSUs) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Summary of non-vested grants (in units)
Balance at the beginning of the period (in units)	26,112
Granted (in units)	1,520
Issued (in units)	(4,357)
Balance at the end of the period (in units)	23,275
Weighted average grant date fair value per unit
Balance at the beginning of the period (in dollars per unit)	$ 36.67
Granted (in dollars per unit)	27.91
Issued (in dollars per unit)	39.18
Balance at the end of the period (in dollars per unit)	$ 35.63
Intrinisic value (in dollars)
Intrinsic value of outstanding grants (in dollars)	$ 647	$ 734