Joint Venture with Peabody Energy (Details)	Jun. 18, 2019 USD ($)
Peabody Energy Corporation \| Senior Notes \| Senior Notes Due 2022
Schedule of Equity Method Investments [Line Items]
Debt instrument stated interest rate	6.00%
Peabody Energy Corporation \| Senior Notes \| Senior Notes Due 2025
Schedule of Equity Method Investments [Line Items]
Debt instrument stated interest rate	6.375%
Implementation Agreement
Schedule of Equity Method Investments [Line Items]
Economic interest held	33.50%
Termination fee payable to non-terminating party	$ 40,000,000
Reduction in contract termination fee payable to non-terminating party	$ 25,000,000
Implementation Agreement \| Peabody Energy Corporation
Schedule of Equity Method Investments [Line Items]
Economic interest held	66.50%

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