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Derivatives and Fair Value Measurements - Narrative (Details)
 3 Months Ended 9 Months Ended 12 Months Ended
Dec. 31, 2021
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Sep. 30, 2021
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Sep. 30, 2020
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Sep. 30, 2021
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Sep. 30, 2020
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Dec. 31, 2022
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Dec. 31, 2021
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Sep. 30, 2021
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Sep. 29, 2021
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Dec. 31, 2020
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Derivatives, Fair Value [Line Items]                    
Derivative, initial margin posted   $ 239,600,000   $ 239,600,000           $ 9,500,000
Proceeds from common stock issuances, net of costs       177,200,000 $ 0          
Net gain on early debt extinguishment   16,000,000.0 $ 0 31,300,000 0          
Margin Deposit Assets   214,300,000   $ 214,300,000           6,500,000
Unobservable Measurement Input, Uncertainty, Description       10            
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset [Abstract]                    
Revenues   679,000,000.0 671,000,000.0 $ 2,053,700,000 2,143,900,000          
Trading and Brokerage Coal Deliveries                    
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset [Abstract]                    
Revenues   $ 55,500,000 (13,000,000.0) $ 97,300,000 (32,100,000)          
Debt for Equity Exchange                    
Derivatives, Fair Value [Line Items]                    
Common stock, shares issued (in shares) | shares   6,700,000   6,700,000       6,700,000 2,200,000  
Net gain on early debt extinguishment   $ 1,000,000   $ 900,000            
6.000% Senior Secured Notes due March 2022 (2022 Notes) | Senior Notes | Debt for Equity Exchange                    
Derivatives, Fair Value [Line Items]                    
Proceeds from common stock issuances, net of costs   6,400,000   37,300,000            
Designated as Hedging Instrument                    
Derivatives, Fair Value [Line Items]                    
Variation margin (received) posted   1,500,000   1,500,000           300,000
Unrealized (loss) gain recognized in income on derivatives   (237,800,000) (15,400,000) (271,500,000) (10,100,000)          
Designated as Hedging Instrument | Net Amount                    
Derivatives, Fair Value [Line Items]                    
Variation margin (received) posted   214,300,000   214,300,000           6,500,000
Subsequent Event | 6.000% Senior Secured Notes due March 2022 (2022 Notes) | Senior Notes | Debt for Equity Exchange                    
Derivatives, Fair Value [Line Items]                    
Common stock, shares issued (in shares) | shares 800,000           800,000      
Foreign currency option contracts                    
Derivatives, Fair Value [Line Items]                    
Notional amount               $ 595,000,000    
Foreign currency option contracts | Designated as Hedging Instrument                    
Derivatives, Fair Value [Line Items]                    
Unrealized (loss) gain recognized in income on derivatives   600,000 700,000 $ (8,300,000) 3,600,000          
Financial trading contracts                    
Derivatives, Fair Value [Line Items]                    
Derivative notional amount (in tonnes) | t       2,900,000            
Financial trading contracts | Designated as Hedging Instrument                    
Derivatives, Fair Value [Line Items]                    
Unrealized (loss) gain recognized in income on derivatives   0 0 $ 800,000 (2,400,000)          
Diesel Fuel Hedge Contracts                    
Derivatives, Fair Value [Line Items]                    
Notional amount               $ 0    
Coal Contract and Physical commodity purchase / sale contracts | Designated as Hedging Instrument                    
Derivatives, Fair Value [Line Items]                    
Unrealized (loss) gain recognized in income on derivatives   238,400,000 $ 16,100,000 263,200,000 $ 13,700,000          
Gain (Loss) on Derivative Instruments | Designated as Hedging Instrument                    
Derivatives, Fair Value [Line Items]                    
Unrealized (loss) gain recognized in income on derivatives   183,000,000                
Forward Contracts | Designated as Hedging Instrument                    
Derivatives, Fair Value [Line Items]                    
Unrealized (loss) gain recognized in income on derivatives   55,000,000                
Forecast                    
Derivatives, Fair Value [Line Items]                    
Price per ton $ 84           $ 84      
Forecast | Foreign currency option contracts                    
Derivatives, Fair Value [Line Items]                    
Derivative, exchange rate floor (in dollars per share) | $ / $ 0.77           0.77      
Derivative, exchange rate cap (in dollars per share) | $ / $ 0.81           0.81      
Forecast | Financial trading contracts                    
Derivatives, Fair Value [Line Items]                    
Derivative notional amount (in tonnes) | t 800,000           2,100,000      
Forecast | Coal Contract to settle in 2022                    
Derivatives, Fair Value [Line Items]                    
Derivative notional amount (in tonnes) | t             1,400,000      
Forecast | Coal Contract to settle in 2023                    
Derivatives, Fair Value [Line Items]                    
Derivative notional amount (in tonnes) | t             700,000      
Forecast | Other Coal Financial Contracts                    
Derivatives, Fair Value [Line Items]                    
Derivative notional amount (in tonnes) | t 200,000         600,000        
Coal Trading                    
Derivatives, Fair Value [Line Items]                    
Derivative, initial margin posted   $ 25,300,000   $ 25,300,000           $ 3,000,000