Intangible Contract Assets and Liabilities Intangible Contract Assets and Liabilities Disclosure (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Finite Lived Intangible Liabilities	$ (8.4)	$ (20.3)
Fresh Start Reporting
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	20.7	70.9
Finite Lived Intangible Liabilities	(61.4)	(90.0)
Finite-Lived Liabilities, Net	(40.7)
Finite-Lived Intangible Assets, Net		(19.1)
Fresh Start Reporting | Coal supply agreements
Finite-Lived Intangible Assets [Line Items]
Finite Lived Intangible Liabilities	(21.4)	(32.9)
Fresh Start Reporting | Take-or-pay contracts
Finite-Lived Intangible Assets [Line Items]
Finite Lived Intangible Liabilities	(40.0)	(57.1)
Finite-Lived Liabilities, Net	(40.0)	(57.1)
Investments and other assets | Fresh Start Reporting
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	20.7	70.9
Finite Lived Intangible Liabilities	0.0	0.0
Finite-Lived Intangible Assets, Net	(20.7)	(70.9)
Accounts payable and accrued expenses | Fresh Start Reporting
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	0.0	0.0
Finite Lived Intangible Liabilities	(8.4)	(20.3)
Finite-Lived Liabilities, Net	(8.4)	(20.3)
Other noncurrent liabilities | Fresh Start Reporting
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	0.0	0.0
Finite Lived Intangible Liabilities	(53.0)	(69.7)
Finite-Lived Liabilities, Net	(53.0)	(69.7)
Coal supply agreements | Fresh Start Reporting
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	20.7	70.9
Finite-Lived Intangible Assets, Net	(0.7)	(38.0)
Take-or-pay contracts | Fresh Start Reporting
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	$ 0.0	$ 0.0