DERIVATIVE INSTRUMENTS - Offsetting Arrangements (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Offsetting Derivative Assets [Abstract]
Gross amounts of recognized assets/liabilities	$ 2,562	$ 1,141
Offsetting Derivative Liabilities [Abstract]
Gross amount offset in the balance sheet	13,695	13,471
Commodity Contract
Offsetting Derivative Assets [Abstract]
Gross amounts of recognized assets/liabilities	2,562	1,141
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	2,562	1,141
Gross amounts not offset in the balance sheet, Financial Instruments	(582)	(399)
Gross amounts not offset in the balance sheet, Cash Collateral Posted	(339)	0
Net amount	1,641	742
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets/liabilities	(3,349)	(5,840)
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	(3,349)	(5,840)
Gross amounts not offset in the balance sheet, Financial Instruments	582	399
Gross amounts not offset in the balance sheet, Cash Collateral Posted	0	5,025
Net amount	(2,767)	(416)
Derivatives - Other
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets/liabilities	(10,346)	(7,631)
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	(10,346)	(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	$ (10,346)	$ (7,631)