DERIVATIVE INSTRUMENTS - FAIR VALUES OF ALL DERIVATIVE INSTRUMENTS (Details) - Derivatives not designated as hedging instruments under GAAP - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Assets	$ 80,893	$ 99,331
Liabilities	74,652	123,348
Commodity contract | Derivatives – Energy Related – Current
Derivatives, Fair Value [Line Items]
Assets	72,391	83,093
Liabilities	60,082	90,708
Commodity contract | Derivatives – Energy Related – Non-Current
Derivatives, Fair Value [Line Items]
Assets	8,502	16,238
Liabilities	4,540	21,697
Interest rate contract | Derivatives - Other - Current
Derivatives, Fair Value [Line Items]
Assets	0	0
Liabilities	681	0
Interest rate contract | Derivatives - Other - Non-Current
Derivatives, Fair Value [Line Items]
Assets	0	0
Liabilities	$ 9,349	$ 10,943