Derivative Commodity Instruments - Offsetting Assets and Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Assets
Gross Amounts Recognized at Fair Value	$ 135,588	$ 1,800
Gross Amounts Offset in the Balance Sheets	(99,953)	(1,800)
Net Amounts Presented in the Balance Sheets	35,635	0
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	35,635	0
Liabilities
Gross Amounts Recognized at Fair Value	211,984	82,065
Gross Amounts Offset in the Balance Sheets	(99,953)	(1,800)
Net Amount Presented in the Balance Sheets	112,031	80,265
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	112,031	80,265
Total Derivatives	(76,396)	(80,265)
Derivative instruments
Assets
Gross Amounts Recognized at Fair Value	116,181	1,758
Gross Amounts Offset in the Balance Sheets	(80,804)	(1,758)
Net Amounts Presented in the Balance Sheets	35,377	0
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	35,377	0
Noncurrent derivative instruments
Assets
Gross Amounts Recognized at Fair Value	19,407	42
Gross Amounts Offset in the Balance Sheets	(19,149)	(42)
Net Amounts Presented in the Balance Sheets	258	0
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	258	0
Derivative instruments
Liabilities
Gross Amounts Recognized at Fair Value	161,800	73,137
Gross Amounts Offset in the Balance Sheets	(80,804)	(1,758)
Net Amount Presented in the Balance Sheets	80,996	71,379
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	80,996	71,379
Noncurrent derivative instruments
Liabilities
Gross Amounts Recognized at Fair Value	50,184	8,928
Gross Amounts Offset in the Balance Sheets	(19,149)	(42)
Net Amount Presented in the Balance Sheets	31,035	8,886
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	$ 31,035	$ 8,886