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Fair Value Measurements - Hierarchy of Fair Value, Assets and Liabilities Measured on Recurring Basis (Details) - Recurring - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Assets:    
Total assets $ 4,792 $ 107,018
Liabilities:    
Total liabilities 28,934 104
Money market funds    
Assets:    
Total assets 144 143
Commodity derivative instruments    
Assets:    
Total assets 4,648 106,875
Liabilities:    
Total liabilities 28,934 104
Level 1    
Assets:    
Total assets 144 143
Liabilities:    
Total liabilities 0 0
Level 1 | Money market funds    
Assets:    
Total assets 144 143
Level 1 | Commodity derivative instruments    
Assets:    
Total assets 0 0
Liabilities:    
Total liabilities 0 0
Level 2    
Assets:    
Total assets 4,648 106,875
Liabilities:    
Total liabilities 28,934 104
Level 2 | Money market funds    
Assets:    
Total assets 0 0
Level 2 | Commodity derivative instruments    
Assets:    
Total assets 4,648 106,875
Liabilities:    
Total liabilities 28,934 104
Level 3    
Assets:    
Total assets 0 0
Liabilities:    
Total liabilities 0 0
Level 3 | Money market funds    
Assets:    
Total assets 0 0
Level 3 | Commodity derivative instruments    
Assets:    
Total assets 0 0
Liabilities:    
Total liabilities $ 0 $ 0