FAIR VALUE MEASUREMENTS - Summary of Changes of Level 3 Financial Assets (Details) - Asset retirement obligations - Level 3 - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, January 1	$ 2,810	$ 2,803	$ 2,471
Accretion	197	207	262
Subsequent revision of estimated obligation	449	0	70
Payments	(1,520)	(200)	0
Balance, December 31	$ 1,936	$ 2,810	$ 2,803