DERIVATIVE INSTRUMENTS - FAIR VALUES OF ALL DERIVATIVE INSTRUMENTS (Details) - Not designated as hedging instruments under GAAP [Member] - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Assets	$ 99,331	$ 99,273
Liabilities	123,348	140,402
Commodity contract [Member] | Derivatives - Energy Related - Current [Member]
Derivatives, Fair Value [Line Items]
Assets	83,093	85,368
Liabilities	90,708	109,744
Commodity contract [Member] | Derivatives - Energy Related - Non-Current [Member]
Derivatives, Fair Value [Line Items]
Assets	16,238	13,905
Liabilities	21,697	19,926
Interest rate contract [Member] | Derivatives - Other [Member]
Derivatives, Fair Value [Line Items]
Assets	0	0
Liabilities	$ 10,943	$ 10,732