DERIVATIVE INSTRUMENTS/HEDGING - Derivative assets and liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Asset
Total gross amounts presented in accompanying balance sheet	$ 1,119	$ 10,745
Liability
Total gross amounts presented in accompanying balance sheet	28,399
Not Designated as Hedging Instrument [Member]
Asset
Total gross amounts presented in accompanying balance sheet	1,119	11,246
Less: gross amounts not offset in the accompanying balance sheet	(1,119)
Net amount:		11,246
Liability
Total gross amounts presented in accompanying balance sheet	31,608
Less: gross amounts not offset in the accompanying balance sheet	(1,119)
Net amount:	30,489
Not Designated as Hedging Instrument [Member] | Oil Contracts [Member] | Current Assets [Member]
Asset
Total gross amounts presented in accompanying balance sheet	1,119	6,774
Not Designated as Hedging Instrument [Member] | Oil Contracts [Member] | Current Liabilities [Member]
Liability
Total gross amounts presented in accompanying balance sheet	17,720
Not Designated as Hedging Instrument [Member] | Oil Contracts [Member] | Non-Current Liabilities [Member]
Liability
Total gross amounts presented in accompanying balance sheet	2,317
Not Designated as Hedging Instrument [Member] | Natural Gas Contracts [Member] | Current Assets [Member]
Asset
Total gross amounts presented in accompanying balance sheet		3,971
Not Designated as Hedging Instrument [Member] | Natural Gas Contracts [Member] | Non-Current Assets [Member]
Asset
Total gross amounts presented in accompanying balance sheet		$ 501
Not Designated as Hedging Instrument [Member] | Natural Gas Contracts [Member] | Current Liabilities [Member]
Liability
Total gross amounts presented in accompanying balance sheet	10,679
Not Designated as Hedging Instrument [Member] | Natural Gas Contracts [Member] | Non-Current Liabilities [Member]
Liability
Total gross amounts presented in accompanying balance sheet	$ 892