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Long-Term Debt - Credit Facility - Additional Information (Detail) - USD ($)
9 Months Ended
Jun. 24, 2019
Sep. 30, 2019
Aug. 06, 2019
Aug. 05, 2019
Dec. 31, 2018
Sep. 30, 2018
Line Of Credit Facility [Line Items]            
Proceeds from Beta Decommissioning Trust account $ 90,000,000 $ 0        
Letters of credit outstanding   1,700,000        
New Revolving Credit Facility [Member]            
Line Of Credit Facility [Line Items]            
Borrowing base   $ 530,000,000        
Maturity date   Nov. 02, 2023        
Borrowing base     $ 530,000,000 $ 425,000,000    
Proceeds from Beta Decommissioning Trust account $ 90,000,000          
Revolving credit facility   $ 278,000,000     $ 294,000,000  
Emergence Credit Facility [Member]            
Line Of Credit Facility [Line Items]            
Revolving credit facility           $ 1,000,000,000