Derivative Commodity Instruments - Offsetting Assets and Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Assets
Gross Amounts Recognized at Fair Value	$ 56,288
Gross Amounts Offset in the Balance Sheets	(7,691)
Net Amounts Presented in the Balance Sheets	48,597
Financial Instruments	0
Cash Collateral Received	0
Net Fair Value Presented in the Balance Sheets	48,597
Liabilities
Gross Amounts Recognized at Fair Value	8,674
Gross Amounts Offset in the Balance Sheets	(7,691)
Net Amount Presented in the Balance Sheets	983
Financial Instruments	0
Cash Collateral Received	0
Net Fair Value Presented in the Balance Sheets	983
Total Derivatives	47,614	$ (68,423)
Current assets
Assets
Gross Amounts Recognized at Fair Value	43,352	1,756
Gross Amounts Offset in the Balance Sheets	(4,289)	(1,706)
Net Amounts Presented in the Balance Sheets	39,063	50
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	39,063	50
Noncurrent assets
Assets
Gross Amounts Recognized at Fair Value	12,936
Gross Amounts Offset in the Balance Sheets	(3,402)
Net Amounts Presented in the Balance Sheets	9,534
Financial Instruments	0
Cash Collateral Received	0
Net Fair Value Presented in the Balance Sheets	9,534
Current Liabilities
Liabilities
Gross Amounts Recognized at Fair Value	4,770	67,173
Gross Amounts Offset in the Balance Sheets	(4,289)	(1,706)
Net Amount Presented in the Balance Sheets	481	65,467
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	481	65,467
Noncurrent Liabilities
Liabilities
Gross Amounts Recognized at Fair Value	3,904	3,006
Gross Amounts Offset in the Balance Sheets	(3,402)	0
Net Amount Presented in the Balance Sheets	502	3,006
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	$ 502	$ 3,006