Long-term Debt - Narrative (Details)		1 Months Ended	3 Months Ended			9 Months Ended		54 Months Ended
	Jan. 29, 2021 USD ($)	Apr. 30, 2018 USD ($)	Dec. 31, 2021 USD ($) shares	Sep. 30, 2021 USD ($) shares	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) shares	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) shares	Oct. 22, 2021 USD ($)	Sep. 29, 2021 shares	Sep. 17, 2021 shares	Jun. 04, 2021 shares	Dec. 31, 2020 USD ($)	Apr. 03, 2017 USD ($)	Feb. 15, 2017 USD ($)
Debt Instrument [Line Items]
Debt issuance cost				$ 45,100,000		$ 45,100,000		$ 45,100,000					$ 42,700,000
Interest expense				45,500,000	$ 34,900,000	143,300,000	$ 102,300,000
Net gain on early debt extinguishment				(16,000,000.0)	0	(31,300,000)	0
Letters of credit outstanding, amount				443,200,000		443,200,000		$ 443,200,000
Payment For Debt Exchange	$ 9,400,000
Payment For Debt Exchange Early Tender Premiums	4,000,000
Professional Fees						10,600,000
Proceeds from common stock issuances, net of costs						177,200,000	0
Open Market Purchase
Debt Instrument [Line Items]
Write off of deferred debt issuance cost				1,600,000		3,700,000
Net gain on early debt extinguishment				15,000,000		26,900,000
Payment for debt extinguishment or debt prepayment cost				$ 46,700,000		$ 85,900,000
At Market Issuance
Debt Instrument [Line Items]
Sale of Stock, Number of Shares Issued in Transaction | shares				9,000,000		17,100,000
Common stock, shares authorized (in shares) | shares				25,000,000		25,000,000		25,000,000			12,500,000	12,500,000
Debt for Equity Exchange
Debt Instrument [Line Items]
Net gain on early debt extinguishment				$ (1,000,000)		$ (900,000)
Common stock, shares issued (in shares) | shares				6,700,000		6,700,000		6,700,000		2,200,000
Subsequent Event
Debt Instrument [Line Items]
Retirement of Debt			$ 17,000,000
Common Stock | At Market Issuance
Debt Instrument [Line Items]
Proceeds from common stock issuances, net of costs				$ 112,100,000		$ 177,200,000
Common Stock | Subsequent Event | At Market Issuance
Debt Instrument [Line Items]
Sale of Stock, Number of Shares Issued in Transaction | shares			3,200,000
Proceeds from common stock issuances, net of costs			$ 43,400,000
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				$ 23,100,000		$ 23,100,000		$ 23,100,000					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]
Proceeds from common stock issuances, net of costs				$ 6,400,000		$ 37,300,000
Senior Notes | 6.000% Senior Secured Notes due March 2022 (2022 Notes) | Subsequent Event | Debt for Equity Exchange
Debt Instrument [Line Items]
Common stock, shares issued (in shares) | shares			800,000
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				$ 9,500,000	9,200,000	$ 27,900,000	27,500,000
Long-term debt				462,400,000		462,400,000		$ 462,400,000					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				2,300,000		19,700,000
Senior Notes | 6.375% Senior Secured Notes due March 2025 (2025 Notes) | Debt for Equity Exchange
Debt Instrument [Line Items]
Proceeds from common stock issuances, net of costs				$ 17,900,000		$ 17,900,000
Senior Notes | 6.375% Senior Secured Notes due March 2025 (2025 Notes) | Subsequent Event | Debt for Equity Exchange
Debt Instrument [Line Items]
Proceeds from common stock issuances, net of costs			$ 19,000,000
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
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				$ 128,800,000		128,800,000		$ 128,800,000					0
Debt Issuance Costs, Gross	5,700,000
Interest and Debt Expense				3,800,000		10,800,000
Paid-in-Kind Interest				800,000		2,400,000
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				$ 22,100,000		$ 40,100,000
Senior Notes | 8.500% Senior Secured Notes due December 2024 (Peabody Notes) | Debt for Equity Exchange
Debt Instrument [Line Items]
Proceeds from common stock issuances, net of costs				$ 5,500,000		$ 5,500,000
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									$ 15,800,000
Debt Instrument, Exchange Offer, Required Purchase, Percentage Of Accreted Value									0.73590
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
Debt Issuance Costs, Gross	5,600,000
Interest and Debt Expense				5,400,000		14,300,000
Term Loan | Successor Credit Agreement
Debt Instrument [Line Items]
Interest expense				$ 3,300,000	3,400,000	10,100,000	12,100,000
Original issue discount and deferred finance costs													10,400,000	$ 37,300,000
Debt instrument, periodic payment						$ 1,000,000
Repayments of debt								$ 500,000,000
Payment for debt extinguishment or debt prepayment cost		$ 46,000,000
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
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
Debt Issuance Costs, Gross	7,100,000
Interest and Debt Expense				5,700,000		14,900,000
Term Loan | Senior Secured Term Loan due 2025, net of original issue discount (Senior Secured Term Loan)
Debt Instrument [Line Items]
Long-term debt				328,700,000		328,700,000		328,700,000					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				38,900,000		56,700,000
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			5,000,000
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
Line of credit | Letter of Credit
Debt Instrument [Line Items]
Proceeds from Issuance of Debt	324,000,000
Debt Issuance Costs, Gross	4,100,000
Interest and Debt Expense				6,000,000		$ 15,900,000
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
Debt Instrument, Covenant, Unrestricted Subsidiaries' Liquidity	200,000,000
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	$ 5,100,000	$ 1,400,000	$ 10,800,000
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
Peabody Notes | Subsequent Event | At Market Issuance
Debt Instrument [Line Items]
Payment for debt extinguishment or debt prepayment cost			3,300,000
Peabody Notes | Subsequent Event | Debt for Equity Exchange
Debt Instrument [Line Items]
Proceeds from common stock issuances, net of costs			$ 12,000,000
Common stock, shares issued (in shares) | shares			1,100,000