DERIVATIVE COMMODITY INSTRUMENTS - Offsetting of Derivative Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets
Gross Amounts Recognized at Fair Value	$ 1,800
Gross Amounts Offset in the Balance Sheets	(1,800)
Net Amounts Presented in the Balance Sheets	0
Financial Instruments	0
Cash Collateral Received	0
Net Fair Value Presented in the Balance Sheets	0
Liabilities
Gross Amounts Recognized at Fair Value	82,065	$ 70,179
Gross Amounts Offset in the Balance Sheets	(1,800)	(1,706)
Net Amounts Presented in the Balance Sheets	80,265	68,473
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	80,265	68,473
Total derivatives	(80,265)	(68,423)
Current Assets
Assets
Gross Amounts Recognized at Fair Value	1,758	1,756
Gross Amounts Offset in the Balance Sheets	(1,758)	(1,706)
Net Amounts Presented in the Balance Sheets	0	50
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	0	50
Noncurrent Assets
Assets
Gross Amounts Recognized at Fair Value	42
Gross Amounts Offset in the Balance Sheets	(42)
Net Amounts Presented in the Balance Sheets	0
Financial Instruments	0
Cash Collateral Received	0
Net Fair Value Presented in the Balance Sheets	0
Current Liabilities
Liabilities
Gross Amounts Recognized at Fair Value	73,137	67,173
Gross Amounts Offset in the Balance Sheets	(1,758)	(1,706)
Net Amounts Presented in the Balance Sheets	71,379	65,467
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	71,379	65,467
Noncurrent Liabilities
Liabilities
Gross Amounts Recognized at Fair Value	8,928	3,006
Gross Amounts Offset in the Balance Sheets	(42)	0
Net Amounts Presented in the Balance Sheets	8,886	3,006
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	$ 8,886	$ 3,006