DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES - Location and Fair Value Amounts of Derivative Instruments Reported in Balance Sheet (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Asset Derivatives
Fair value of derivative instruments, assets	$ 879
Liability Derivatives
Fair value of derivative instruments, liabilities	12,054
Not Designated as Hedging Instrument | Commodity contracts
Asset Derivatives
Fair value of derivative instruments, assets	900
Liability Derivatives
Fair value of derivative instruments, liabilities	12,100
Not Designated as Hedging Instrument | Commodity contracts | Current assets: Fair value of derivative contracts
Asset Derivatives
Fair value of derivative instruments, assets	900
Not Designated as Hedging Instrument | Commodity contracts | Long-term assets: Fair value of derivative contracts
Asset Derivatives
Fair value of derivative instruments, assets	0
Not Designated as Hedging Instrument | Commodity contracts | Current liabilities: Fair value of derivative contracts
Liability Derivatives
Fair value of derivative instruments, liabilities	9,000
Not Designated as Hedging Instrument | Commodity contracts | Long-term liabilities: Fair value of derivative contracts
Liability Derivatives
Fair value of derivative instruments, liabilities	$ 3,100