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Summary of Significant Accounting Policies - Narrative (Details)
12 Months Ended
Dec. 31, 2020
USD ($)
Institution
Segment
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Properties and Equipment      
Number of operating segments | Segment 1    
Number of financial institutions | Institution 1    
Restricted cash $ 11,578,000 $ 13,556,000  
Amortization of unproved properties included in depreciation, depletion and amortization 8,200,000 32,600,000 $ 82,300,000
Assets legally restricted for purposes of settling asset retirement obligations 0 0  
Finance leases 0 0  
Accounts Payable      
Properties and Equipment      
Book overdraft $ 0 $ 0  
Buildings | Minimum      
Properties and Equipment      
Estimated useful life 25 years    
Buildings | Maximum      
Properties and Equipment      
Estimated useful life 40 years    
Other Assets | Minimum      
Properties and Equipment      
Estimated useful life 3 years    
Other Assets | Maximum      
Properties and Equipment      
Estimated useful life 25 years    
Customer One | Sales Revenue, Net | Customer      
Concentration Risk [Line Items]      
Percentage of Total Sales 21.00% 17.00% 20.00%
Customer Two | Sales Revenue, Net | Customer      
Concentration Risk [Line Items]      
Percentage of Total Sales 16.00% 16.00% 11.00%
Customer Three | Sales Revenue, Net | Customer      
Concentration Risk [Line Items]      
Percentage of Total Sales 12.00%