Derivative Commodity Instruments - Offsetting Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets
Gross Amounts Recognized at Fair Value	$ 59,047	$ 1,800
Gross Amounts Offset in the Balance Sheets	(54,821)	(1,800)
Net Amounts Presented in the Balance Sheets	4,226	0
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	4,226	0
Liabilities
Gross Amounts Recognized at Fair Value	285,050	82,065
Gross Amounts Offset in the Balance Sheets	(54,821)	(1,800)
Net Amount Presented in the Balance Sheets	230,229	80,265
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	230,229	80,265
Total Derivatives	(226,003)	(80,265)
Derivative instruments
Assets
Gross Amounts Recognized at Fair Value	55,455	1,758
Gross Amounts Offset in the Balance Sheets	(51,229)	(1,758)
Net Amounts Presented in the Balance Sheets	4,226	0
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	4,226	0
Noncurrent derivative instruments
Assets
Gross Amounts Recognized at Fair Value	3,592	42
Gross Amounts Offset in the Balance Sheets	(3,592)	(42)
Net Amounts Presented in the Balance Sheets	0	0
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	0	0
Derivative instruments
Liabilities
Gross Amounts Recognized at Fair Value	224,001	73,137
Gross Amounts Offset in the Balance Sheets	(51,229)	(1,758)
Net Amount Presented in the Balance Sheets	172,772	71,379
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	172,772	71,379
Noncurrent derivative instruments
Liabilities
Gross Amounts Recognized at Fair Value	61,049	8,928
Gross Amounts Offset in the Balance Sheets	(3,592)	(42)
Net Amount Presented in the Balance Sheets	57,457	8,886
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	$ 57,457	$ 8,886