Derivative Commodity Instruments - Offsetting Assets and Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Assets
Gross Amounts Recognized at Fair Value	$ 40,206	$ 1,800
Gross Amounts Offset in the Balance Sheets	(31,961)	(1,800)
Net Amounts Presented in the Balance Sheets	8,245	0
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	8,245	0
Liabilities
Gross Amounts Recognized at Fair Value	101,002	82,065
Gross Amounts Offset in the Balance Sheets	(31,961)	(1,800)
Net Amount Presented in the Balance Sheets	69,041	80,265
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	69,041	80,265
Total Derivatives	(60,796)	(80,265)
Current assets
Assets
Gross Amounts Recognized at Fair Value	31,420	1,758
Gross Amounts Offset in the Balance Sheets	(26,436)	(1,758)
Net Amounts Presented in the Balance Sheets	4,984	0
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	4,984	0
Noncurrent assets
Assets
Gross Amounts Recognized at Fair Value	8,786	42
Gross Amounts Offset in the Balance Sheets	(5,525)	(42)
Net Amounts Presented in the Balance Sheets	3,261	0
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	3,261	0
Current Liabilities
Liabilities
Gross Amounts Recognized at Fair Value	83,914	73,137
Gross Amounts Offset in the Balance Sheets	(26,436)	(1,758)
Net Amount Presented in the Balance Sheets	57,478	71,379
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	57,478	71,379
Noncurrent Liabilities
Liabilities
Gross Amounts Recognized at Fair Value	17,088	8,928
Gross Amounts Offset in the Balance Sheets	(5,525)	(42)
Net Amount Presented in the Balance Sheets	11,563	8,886
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	$ 11,563	$ 8,886