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Fair Value Measurements - Hierarchy of Fair Value, Assets and Liabilities Measured on Recurring Basis (Details) - Fair Value, Measurements, Recurring [Member] - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Assets:    
Total assets $ 143 $ 495
Liabilities:    
Total liabilities 213,146 135,567
Money market funds [Member]    
Assets:    
Total assets 143 142
Commodity derivative instruments [Member]    
Assets:    
Total assets   353
Liabilities:    
Total liabilities 213,146 135,567
Level 1 [Member]    
Assets:    
Total assets 143 142
Liabilities:    
Total liabilities 0 0
Level 1 [Member] | Money market funds [Member]    
Assets:    
Total assets 143 142
Level 1 [Member] | Commodity derivative instruments [Member]    
Assets:    
Total assets   0
Liabilities:    
Total liabilities 0 0
Level 2 [Member]    
Assets:    
Total assets 0 353
Liabilities:    
Total liabilities 213,146 135,567
Level 2 [Member] | Money market funds [Member]    
Assets:    
Total assets 0 0
Level 2 [Member] | Commodity derivative instruments [Member]    
Assets:    
Total assets   353
Liabilities:    
Total liabilities 213,146 135,567
Level 3 [Member]    
Assets:    
Total assets 0 0
Liabilities:    
Total liabilities 0 0
Level 3 [Member] | Money market funds [Member]    
Assets:    
Total assets 0 0
Level 3 [Member] | Commodity derivative instruments [Member]    
Assets:    
Total assets   0
Liabilities:    
Total liabilities $ 0 $ 0