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Long-term Debt - Narrative (Details) |  | 1 Months Ended | 3 Months Ended | 6 Months Ended | 51 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Jan. 29, 2021 USD ($) | Apr. 30, 2018 USD ($) | Sep. 30, 2021 USD ($) shares | Jun. 30, 2021 USD ($) shares | Jun. 30, 2020 USD ($) | Jun. 30, 2021 USD ($) shares | Jun. 30, 2020 USD ($) | Jun. 30, 2021 USD ($) shares | Jul. 07, 2021 USD ($) | Dec. 31, 2020 USD ($) | Apr. 03, 2017 USD ($) | Feb. 15, 2017 USD ($)
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt issuance cost |  |  |  | $ 51,100,000 |  | $ 51,100,000 |  | $ 51,100,000 |  | $ 42,700,000 |  |
Interest expense |  |  |  | 45,400,000 | $ 34,300,000 | 97,800,000 | $ 67,400,000 |  |  |  |  |
Net gain on early debt extinguishment |  |  |  | (11,800,000) | 0 | (15,300,000) | 0 |  |  |  |  |
Letters of credit outstanding, amount |  |  |  | 435,400,000 |  | 435,400,000 |  | 435,400,000 |  |  |  |
Payment For Debt Exchange | $ 9,400,000 |  |  |  |  |  |  |  |  |  |  |
Payment For Debt Exchange Early Tender Premiums | 4,000,000.0 |  |  |  |  |  |  |  |  |  |  |
Professional Fees |  |  |  |  |  | 10,600,000 |  |  |  |  |  |
Proceeds from common stock issuances, net of costs |  |  |  |  |  | 65,100,000 | 0 |  |  |  |  |
Open Market Purchase |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net gain on early debt extinguishment |  |  |  |  |  | 11,900,000 |  |  |  |  |  |
Payment for debt extinguishment or debt prepayment cost |  |  |  | 39,200,000 |  | 39,200,000 |  |  |  |  |  |
Debt Issuance Costs, Gross |  |  |  | $ 2,100,000 |  | $ 2,100,000 |  | $ 2,100,000 |  |  |  |
At Market Issuance |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Sale of Stock, Number of Shares Issued in Transaction | shares |  |  |  | 8,100,000 |  | 8,100,000 |  |  |  |  |  |
Common stock, shares authorized (in shares) | shares |  |  |  | 12,500,000 |  | 12,500,000 |  | 12,500,000 |  |  |  |
Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Retirement of Debt |  |  | $ 49,500,000 |  |  |  |  |  |  |  |  |
Common Stock | At Market Issuance |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from common stock issuances, net of costs |  |  |  | $ 65,100,000 |  | $ 65,100,000 |  |  |  |  |  |
Common Stock | Subsequent Event | At Market Issuance |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Sale of Stock, Number of Shares Issued in Transaction | shares |  |  | 2,700,000 |  |  |  |  |  |  |  |  |
Proceeds from common stock issuances, net of costs |  |  | $ 21,500,000 |  |  |  |  |  |  |  |  |
Senior Notes | Subsequent Event | At Market Issuance |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Payment for debt extinguishment or debt prepayment cost |  |  | 30,200,000 |  |  |  |  |  |  |  |  |
Senior Notes | 6.000% Senior Secured Notes due March 2022 (2022 Notes) |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  | 6.00% |  | 6.00% |  | 6.00% |  |  |  |
Long-term debt |  |  |  | $ 29,500,000 |  | $ 29,500,000 |  | $ 29,500,000 |  | 459,000,000.0 |  |
Debt Instrument Exchanged, Aggregate Principal Amount | 398,700,000 |  |  |  |  |  |  |  |  |  |  |
Senior Notes | 6.000% Senior Secured Notes due March 2022 (2022 Notes) | Debt for Equity Exchange |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net gain on early debt extinguishment |  |  |  | (100,000) |  | (100,000) |  |  |  |  |  |
Proceeds from common stock issuances, net of costs |  |  |  | $ 30,900,000 |  | $ 30,900,000 |  |  |  |  |  |
Common stock, shares issued (in shares) | shares |  |  |  | 4,500,000 |  | 4,500,000 |  | 4,500,000 |  |  |  |
Senior Notes | 6.000% Senior Secured Notes due March 2022 (2022 Notes) | Subsequent Event | Debt for Equity Exchange |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from common stock issuances, net of costs |  |  | $ 4,000,000.0 |  |  |  |  |  |  |  |  |
Common stock, shares issued (in shares) | shares |  |  | 500,000 |  |  |  |  |  |  |  |  |
Senior Notes | 6.375% Senior Secured Notes due March 2025 (2025 Notes) |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  | 6.375% |  | 6.375% |  | 6.375% |  |  |  |
Interest expense |  |  |  | $ 9,200,000 | 9,200,000 | $ 18,400,000 | 18,300,000 |  |  |  |  |
Long-term debt |  |  |  | 482,600,000 |  | 482,600,000 |  | $ 482,600,000 |  | 500,000,000.0 |  |
Debt Issuance Costs, Gross |  |  |  |  |  |  |  |  |  |  |  | $ 25,100,000
Senior Notes | 6.375% Senior Secured Notes due March 2025 (2025 Notes) | Open Market Purchase |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Retirement of Debt |  |  |  | $ 17,400,000 |  | $ 17,400,000 |  |  |  |  |  |
Senior Notes | 6.375% Senior Secured Notes due March 2025 (2025 Notes) | Subsequent Event | At Market Issuance |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Retirement of Debt |  |  | $ 1,500,000 |  |  |  |  |  |  |  |  |
Senior Notes | 6.375% Senior Secured Notes due March 2025 (2025 Notes) | Domestic |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Collateral, capital stock, percent |  |  |  | 100.00% |  | 100.00% |  | 100.00% |  |  |  |
Senior Notes | 6.375% Senior Secured Notes due March 2025 (2025 Notes) | Export |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Collateral, non-voting capital stock, percent |  |  |  | 100.00% |  | 100.00% |  | 100.00% |  |  |  |
Senior Notes | 6.375% Senior Secured Notes due March 2025 (2025 Notes) | Effect of Plan |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Principal amount |  |  |  |  |  |  |  |  |  |  |  | $ 500,000,000.0
Senior Notes | 8.500% Senior Secured Notes due December 2024 (Peabody Notes) |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Principal amount | 195,100,000 |  |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  | 8.50% |  | 8.50% |  | 8.50% |  |  |  |
Net gain on early debt extinguishment |  |  |  |  |  | $ 3,500,000 |  |  |  |  |  |
Long-term debt |  |  |  | $ 156,300,000 |  | 156,300,000 |  | $ 156,300,000 |  | 0 |  |
Debt Issuance Costs, Gross | 5,700,000 |  |  |  |  |  |  |  |  |  |  |
Interest and Debt Expense |  |  |  | 3,900,000 |  | 7,000,000.0 |  |  |  |  |  |
Paid-in-Kind Interest |  |  |  | 800,000 |  | 1,600,000 |  |  |  |  |  |
Debt Instrument, Exchange Offer, Required Purchase Amount |  |  |  | $ 22,400,000 |  | $ 22,400,000 |  | $ 22,400,000 |  |  |  |
Debt Instrument, Exchange Offer, Required Purchase, Percentage Of Accreted Value |  |  |  | 0.80 |  | 0.80 |  | 0.80 |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage In Cash |  |  |  | 6.00% |  | 6.00% |  | 6.00% |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage, Paid-In-Kind |  |  |  | 2.50% |  | 2.50% |  | 2.50% |  |  |  |
Senior Notes | 8.500% Senior Secured Notes due December 2024 (Peabody Notes) | Open Market Purchase |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Retirement of Debt |  |  |  | $ 18,000,000.0 |  | $ 18,000,000.0 |  |  |  |  |  |
Senior Notes | 8.500% Senior Secured Notes due December 2024 (Peabody Notes) | Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Exchange Offer, Required Purchase Amount |  |  |  |  |  |  |  |  | $ 13,300,000 |  |  |
Debt Instrument, Exchange Offer, Required Purchase, Percentage Of Accreted Value |  |  |  |  |  |  |  |  | 0.73840 |  |  |
Senior Notes | 10.000% Senior Secured Notes due December 2024 (Co-Issuer Notes) |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Principal amount | $ 193,900,000 |  |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  | 10.00% |  | 10.00% |  | 10.00% |  |  |  |
Long-term debt |  |  |  | $ 193,900,000 |  | $ 193,900,000 |  | $ 193,900,000 |  | 0 |  |
Debt Instrument, Secured Liens Term, Percentage Of Capital Stock | 100.00% |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Covenant, Liquidity Attributable To Co-Issuers | $ 60,000,000.0 |  |  |  |  |  |  |  |  |  |  |
Debt Issuance Costs, Gross | 5,600,000 |  |  |  |  |  |  |  |  |  |  |
Interest and Debt Expense |  |  |  | 5,300,000 |  | 8,900,000 |  |  |  |  |  |
Term Loan | Successor Credit Agreement |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Interest expense |  |  |  | $ 3,500,000 | 3,800,000 | 6,800,000 | 8,700,000 |  |  |  |  |
Original issue discount and deferred finance costs |  |  |  |  |  |  |  |  |  | 10,400,000 | $ 37,300,000 |
Debt instrument, periodic payment |  |  |  |  |  | $ 1,000,000.0 |  |  |  |  |  |
Repayments of debt |  |  |  |  |  |  |  | $ 500,000,000.0 |  |  |  |
Payment for debt extinguishment or debt prepayment cost |  | $ 46,000,000.0 |  |  |  |  |  |  |  |  |  |
Term Loan | Successor Credit Agreement | London Interbank Offered Rate (LIBOR) [Member] |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Basis spread on variable rate |  |  |  |  |  | 2.75% |  |  |  |  |  |
Term Loan | Successor Credit Agreement | Effect of Plan |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Principal amount |  |  |  |  |  |  |  |  |  |  | $ 950,000,000.0 |
Term Loan | Senior Secured Term Loan due 2024 (Co-Issuer Term Loans) |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  | 10.00% |  | 10.00% |  | 10.00% |  |  |  |
Long-term debt |  |  |  | $ 206,000,000.0 |  | $ 206,000,000.0 |  | $ 206,000,000.0 |  | 0 |  |
Proceeds from Issuance of Debt | 206,000,000.0 |  |  |  |  |  |  |  |  |  |  |
Debt Issuance Costs, Gross | 7,100,000 |  |  |  |  |  |  |  |  |  |  |
Interest and Debt Expense |  |  |  | 5,500,000 |  | 9,200,000 |  |  |  |  |  |
Term Loan | Senior Secured Term Loan due 2025, net of original issue discount (Senior Secured Term Loan) |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt |  |  |  | 368,600,000 |  | 368,600,000 |  | 368,600,000 |  | 388,200,000 |  |
Term Loan | Senior Secured Term Loan due 2025, net of original issue discount (Senior Secured Term Loan) | Open Market Purchase |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Retirement of Debt |  |  |  | 17,800,000 |  | 17,800,000 |  |  |  |  |  |
Term Loan | Senior Secured Term Loan due 2025, net of original issue discount (Senior Secured Term Loan) | Subsequent Event | At Market Issuance |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Retirement of Debt |  |  | $ 44,000,000.0 |  |  |  |  |  |  |  |  |
Line of credit | Revolving credit facility |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt |  |  |  | 0 |  | 0 |  | $ 0 |  | $ 216,000,000.0 |  |
Repayments of lines of credit | 10,000,000.0 |  |  |  |  |  |  |  |  |  |  |
Line of credit | Letter of Credit |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from Issuance of Debt | 324,000,000.0 |  |  |  |  |  |  |  |  |  |  |
Debt Issuance Costs, Gross | 4,100,000 |  |  |  |  |  |  |  |  |  |  |
Interest and Debt Expense |  |  |  | 5,800,000 |  | $ 9,900,000 |  |  |  |  |  |
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage |  |  |  |  |  | 6.00% |  |  |  |  |  |
Line of Credit Facility, Commitment Fee Percentage |  |  |  |  |  | 0.50% |  |  |  |  |  |
Debt Instrument, Covenant, Aggregate Liquidity At End Of Each Quarter | 125,000,000.0 |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Covenant, Unrestricted Subsidiaries' Liquidity | 200,000,000.0 |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Refinancing Transactions, Principal Repurchase Basis | 4 |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Refinancing Transactions, Principal Amount Basis For Purchase | $ 1 |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Refinancing Transactions, Debt Repurchases Period | 30 days |  |  |  |  |  |  |  |  |  |  |
Revolving credit facility | 2017 Revolver |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Interest expense |  |  |  | $ 0 | $ 4,000,000.0 | $ 1,400,000 | $ 5,700,000 |  |  |  |  |
2019 Revolver commitments, matures 2020 | $ 0 |  |  |  |  |  |  |  |  |  |  |
6.000% Senior Secured Notes due March 2022 (2022 Notes) |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net gain on early debt extinguishment |  |  |  |  |  | $ 0 |  |  |  |  |  |

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