DERIVATIVE INSTRUMENTS - OFFSETTING ARRANGEMENTS (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Commodity contract
Offsetting Derivative Assets [Abstract]
Gross amounts of recognized assets/liabilities	$ 80,893	$ 99,331
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	80,893	99,331
Gross amounts not offset in the balance sheet, Financial Instruments	(38,809)	(35,491)
Gross amounts not offset in the balance sheet, Cash Collateral Posted	(3,474)	0
Net amount	38,610	63,840
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets/liabilities	(64,622)	(112,405)
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	(64,622)	(112,405)
Gross amounts not offset in the balance sheet, Financial Instruments	38,809	35,491
Gross amounts not offset in the balance sheet, Cash Collateral Posted	0	23,045
Net amount	(25,813)	(53,869)
Interest rate contract
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets/liabilities	(10,030)	(10,943)
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	(10,030)	(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	$ (10,030)	$ (10,943)