FAIR VALUE MEASUREMENTS - Unobservable Inputs (Level 3) (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Fair Value Measurements Using Unobservable Inputs (Level 3)
Balance, beginning of period	$ (12,600)	$ (2,300)
Accretion expense (recognized in general and administrative expenses)	(1,300)	(706)
Settlement (payment) of liability	0	3,006
Contingent consideration attributable to Creehan acquisition	0	(12,600)
Total	$ (13,900)	$ (12,600)