DERIVATIVE INSTRUMENTS - Derivative Assets and Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Asset
Total gross amounts presented in the balance sheet	$ 72,943	$ 15,151
Liability
Total gross amounts presented in the balance sheet	90,480	42,066
Not Designated as Hedging Instrument
Asset
Total gross amounts presented in the balance sheet	75,273	17,237
Less: gross amounts not offset in the balance sheet	(68,377)	(17,237)
Net amount	6,896	0
Liability
Total gross amounts presented in the balance sheet	101,991	46,334
Less: gross amounts not offset in the balance sheet	(68,377)	(17,237)
Net amount	33,614	29,097
Not Designated as Hedging Instrument \| Oil contracts \| Current assets — Derivative instruments
Asset
Total gross amounts presented in the balance sheet	57,768
Not Designated as Hedging Instrument \| Oil contracts \| Non-current assets — Derivative instruments
Asset
Total gross amounts presented in the balance sheet	1,106
Not Designated as Hedging Instrument \| Oil contracts \| Current liabilities — Derivative instruments
Liability
Total gross amounts presented in the balance sheet	88,814	42,066
Not Designated as Hedging Instrument \| Oil contracts \| Non-current liabilities — Derivative instruments
Liability
Total gross amounts presented in the balance sheet	11,237	4,268
Not Designated as Hedging Instrument \| Gas contracts \| Current assets — Derivative instruments
Asset
Total gross amounts presented in the balance sheet	15,175	15,151
Not Designated as Hedging Instrument \| Gas contracts \| Non-current assets — Derivative instruments
Asset
Total gross amounts presented in the balance sheet	1,224	$ 2,086
Not Designated as Hedging Instrument \| Gas contracts \| Current liabilities — Derivative instruments
Liability
Total gross amounts presented in the balance sheet	1,666
Not Designated as Hedging Instrument \| Gas contracts \| Non-current liabilities — Derivative instruments
Liability
Total gross amounts presented in the balance sheet	$ 274

Fair value of liability associated with financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.

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