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Fair Value Measurements - Hierarchy of Fair Value, Assets and Liabilities Measured on Recurring Basis (Detail) - Recurring - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Assets:    
Total assets $ 107,018 $ 495
Total liabilities 104 135,567
Money market funds    
Assets:    
Total assets 143 142
Commodity derivative instruments    
Assets:    
Total assets 106,875 353
Total liabilities 104 135,567
Level 1    
Assets:    
Total assets 143 142
Total liabilities 0 0
Level 1 | Money market funds    
Assets:    
Total assets 143 142
Level 1 | Commodity derivative instruments    
Assets:    
Total assets 0 0
Total liabilities 0 0
Level 2    
Assets:    
Total assets 106,875 353
Total liabilities 104 135,567
Level 2 | Money market funds    
Assets:    
Total assets 0 0
Level 2 | Commodity derivative instruments    
Assets:    
Total assets 106,875 353
Total liabilities 104 135,567
Level 3    
Assets:    
Total assets 0 0
Total liabilities 0 0
Level 3 | Money market funds    
Assets:    
Total assets 0 0
Level 3 | Commodity derivative instruments    
Assets:    
Total assets 0 0
Total liabilities $ 0 $ 0