DERIVATIVE INSTRUMENTS - OFFSETTING ARRANGEMENTS (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Offsetting Derivative Assets [Abstract]
Gross amounts of recognized assets/liabilities	$ 77,304	$ 99,331
Offsetting Derivative Liabilities [Abstract]
Gross amount offset in the balance sheet	89,636	123,348
Commodity Contract
Offsetting Derivative Assets [Abstract]
Gross amounts of recognized assets/liabilities	77,304	99,331
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	77,304	99,331
Gross amounts not offset in the balance sheet, Financial Instruments	(15,597)	(35,491)
Gross amounts not offset in the balance sheet, Cash Collateral Posted	0	0
Net amount	61,707	63,840
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets/liabilities	(76,518)	(112,405)
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	(76,518)	(112,405)
Gross amounts not offset in the balance sheet, Financial Instruments	15,597	35,491
Gross amounts not offset in the balance sheet, Cash Collateral Posted	22,020	23,045
Net amount	(38,901)	(53,869)
Other Contract
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets/liabilities	(13,118)	(10,943)
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	(13,118)	(10,943)
Gross amounts not offset in the balance sheet, Financial Instruments	0	0
Gross amounts not offset in the balance sheet, Cash Collateral Posted	0	0
Net amount	$ (13,118)	$ (10,943)