Intangible Contract Assets and Liabilities - Intangible Contract Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Liabilities	$ (4.4)	$ (8.4)
Fresh Start Reporting
Finite-Lived Intangible Assets [Line Items]
Assets	7.9	20.7
Liabilities	(52.0)	(61.4)
Net Total	(44.1)	(40.7)
Fresh Start Reporting | Coal supply agreements
Finite-Lived Intangible Assets [Line Items]
Liabilities	(17.3)	(21.4)
Fresh Start Reporting | Take-or-pay contracts
Finite-Lived Intangible Assets [Line Items]
Liabilities	(34.7)	(40.0)
Net Total	(34.7)	(40.0)
Investments and other assets | Fresh Start Reporting
Finite-Lived Intangible Assets [Line Items]
Assets	7.9	20.7
Liabilities	0.0	0.0
Net Total	7.9	20.7
Accounts payable and accrued expenses | Fresh Start Reporting
Finite-Lived Intangible Assets [Line Items]
Assets	0.0	0.0
Liabilities	(4.4)	(8.4)
Net Total	(4.4)	(8.4)
Other noncurrent liabilities | Fresh Start Reporting
Finite-Lived Intangible Assets [Line Items]
Assets	0.0	0.0
Liabilities	(47.6)	(53.0)
Net Total	(47.6)	(53.0)
Coal supply agreements | Fresh Start Reporting
Finite-Lived Intangible Assets [Line Items]
Assets	7.9	20.7
Net Total	(9.4)	(0.7)
Take-or-pay contracts | Fresh Start Reporting
Finite-Lived Intangible Assets [Line Items]
Assets	$ 0.0	$ 0.0