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Intangible Contract Assets and Liabilities - Textuals (Details) - USD ($)
3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Finite-Lived Intangible Liabilities, Net, Amortization Expense, Fiscal Year Maturity [Abstract]                    
Asset impairment charges $ 1,418,100,000 $ 0 $ 1,418,100,000 $ 0            
Contract-Based Intangible Assets [Member]                    
Finite-Lived Intangible Liabilities, Net, Amortization Expense, Fiscal Year Maturity [Abstract]                    
Asset impairment charges 4,500,000   4,500,000              
Coal supply agreements                    
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract]                    
Presented periods amortization 1,300,000 6,800,000 3,700,000 11,600,000            
Take-or-pay contracts                    
Finite-Lived Intangible Liabilities, Net, Amortization Expense, Fiscal Year Maturity [Abstract]                    
Presented periods amortization $ 2,700,000 $ 5,600,000 $ 5,300,000 $ 11,200,000            
Scenario, Forecast | Coal supply agreements                    
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract]                    
Remainder of 2020                   $ 1,000,000
2021                 $ 1,000,000  
2022               $ (2,000,000)    
2023             $ 2,000,000.0      
2024           $ 2,000,000.0        
2025 and thereafter         $ 3,000,000          
Scenario, Forecast | Take-or-pay contracts                    
Finite-Lived Intangible Liabilities, Net, Amortization Expense, Fiscal Year Maturity [Abstract]                    
Remainder of 2020                   $ 3,000,000
2021                 $ 4,000,000  
2022               $ 3,000,000    
2023             $ 3,000,000      
2024           $ 3,000,000        
2025 and thereafter         $ 18,000,000