DERIVATIVE INSTRUMENTS/HEDGING - Derivative assets and liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Asset
Total gross amounts presented in accompanying balance sheet	$ 11,868	$ 10,745
Liability
Total gross amounts presented in accompanying balance sheet	3,812
Not Designated as Hedging Instrument [Member]
Asset
Total gross amounts presented in accompanying balance sheet	12,290	11,246
Less: gross amounts not offset in the accompanying balance sheet	(1,802)
Net amount:	10,488	11,246
Liability
Total gross amounts presented in accompanying balance sheet	5,684
Less: gross amounts not offset in the accompanying balance sheet	(1,802)
Net amount:	3,882
Not Designated as Hedging Instrument [Member] | Oil Contracts [Member] | Current Assets [Member]
Asset
Total gross amounts presented in accompanying balance sheet	5,132	6,774
Not Designated as Hedging Instrument [Member] | Oil Contracts [Member] | Current Liabilities [Member]
Liability
Total gross amounts presented in accompanying balance sheet	3,812
Not Designated as Hedging Instrument [Member] | Oil Contracts [Member] | Non-Current Liabilities [Member]
Liability
Total gross amounts presented in accompanying balance sheet	1,776
Not Designated as Hedging Instrument [Member] | Natural Gas Contracts [Member] | Current Assets [Member]
Asset
Total gross amounts presented in accompanying balance sheet	6,736	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	422	$ 501
Not Designated as Hedging Instrument [Member] | Natural Gas Contracts [Member] | Non-Current Liabilities [Member]
Liability
Total gross amounts presented in accompanying balance sheet	$ 96