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Equity Method Investments (Details)
3 Months Ended 9 Months Ended
Sep. 30, 2021
USD ($)
Sep. 30, 2020
USD ($)
Sep. 30, 2021
USD ($)
Sep. 30, 2020
USD ($)
Sep. 30, 2021
AUD ($)
Dec. 31, 2020
USD ($)
Schedule of Equity Method Investments [Line Items]            
Equity method investments $ 26,800,000   $ 26,800,000     $ 24,600,000
(Loss) income from equity method investments $ 15,800,000 $ (10,600,000) 11,400,000 $ (25,700,000)    
Cash receipts from Middlemount Coal Pty Ltd and other related parties     8,400,000 0    
Middlemount Mine            
Schedule of Equity Method Investments [Line Items]            
Cash receipts from Middlemount Coal Pty Ltd and other related parties     $ (7,600,000) 0    
Equity interest percentage of revolving loans limit 50.00%   50.00%   50.00%  
Revolving loan limit         $ 160,000,000  
Financing receivable, stated interest rate (in percent)     10.00%      
Intercompany loans, carrying value $ 35,700,000   $ 35,700,000     $ 46,200,000
Middlemount Mine            
Schedule of Equity Method Investments [Line Items]            
(Loss) income from equity method investments $ 15,800,000 $ (10,600,000) $ 11,400,000 $ (25,700,000)