DERIVATIVE INSTRUMENTS - OFFSETTING ARRANGEMENTS (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Offsetting Derivative Assets [Abstract]
Gross amounts of recognized assets/liabilities	$ 4,127	$ 1,141
Offsetting Derivative Liabilities [Abstract]
Gross amount offset in the balance sheet	15,060	13,471
Commodity Contract
Offsetting Derivative Assets [Abstract]
Gross amounts of recognized assets/liabilities	4,127	1,141
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	4,127	1,141
Gross amounts not offset in the balance sheet, Financial Instruments	(933)	(399)
Gross amounts not offset in the balance sheet, Cash Collateral Posted	(1,801)	0
Net amount	1,393	742
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets/liabilities	(4,697)	(5,840)
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	(4,697)	(5,840)
Gross amounts not offset in the balance sheet, Financial Instruments	933	399
Gross amounts not offset in the balance sheet, Cash Collateral Posted	0	5,025
Net amount	(3,764)	(416)
Other Contract
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets/liabilities	(10,363)	(7,631)
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	(10,363)	(7,631)
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	$ (10,363)	$ (7,631)