FAIR VALUE MEASUREMENTS - Narrative (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended			12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Fair value measurements
Non-cash impairment charge					$ 0	$ 23,365	$ 169,002
Natural Gas Storage
Fair value measurements
Non-cash impairment charge	$ 2,500	$ (5,400)	$ 26,300	$ 169,000		23,400
Level 2 | Fair Value, Measurements, Recurring | Physical fixed price derivative contracts
Fair value measurements
Financial assets	44,912				27,676	44,912
Level 2 fixed-price contracts liability, CVA	$ (100)				$ (200)	(100)
Level 3 | Fair Value, Measurements, Non-Recurring | Natural Gas Storage
Fair value measurements
Non-cash impairment charge						$ 23,400	$ 169,000