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Derivatives and Fair Value Measurements - Narrative (Details) | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Dec. 31, 2021 USD ($) t $ / $ shares | Sep. 30, 2021 USD ($) shares | Sep. 30, 2020 USD ($) | Sep. 30, 2021 USD ($) t shares | Sep. 30, 2020 USD ($) | Dec. 31, 2022 t | Dec. 31, 2021 USD ($) t $ / $ shares | Sep. 30, 2021 AUD ($) shares | Sep. 29, 2021 shares | Dec. 31, 2020 USD ($)
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

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