Intangible Contract Assets and Liabilities Intangible Contract Assets and Liabilities Disclosure (Details) - Successor - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Finite Lived Intangible Liabilities	$ (20.3)	$ (27.6)
Fresh Start Reporting
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	70.9	177.2
Finite Lived Intangible Liabilities	(90.0)	(133.4)
Finite-Lived Liabilities, Net	(19.1)
Finite-Lived Intangible Assets, Net		43.8
Fresh Start Reporting | Coal supply agreements
Finite-Lived Intangible Assets [Line Items]
Finite Lived Intangible Liabilities	(32.9)	(42.7)
Fresh Start Reporting | Take-or-pay contracts
Finite-Lived Intangible Assets [Line Items]
Finite Lived Intangible Liabilities	(57.1)	(90.7)
Finite-Lived Liabilities, Net	(57.1)	(90.7)
Investments and other assets | Fresh Start Reporting
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	70.9	177.2
Finite Lived Intangible Liabilities	0.0	0.0
Finite-Lived Intangible Assets, Net	70.9	177.2
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	(20.3)	(27.6)
Finite-Lived Liabilities, Net	(20.3)	(27.6)
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	(69.7)	(105.8)
Finite-Lived Liabilities, Net	(69.7)	(105.8)
Coal supply agreements | Fresh Start Reporting
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	70.9	177.2
Finite-Lived Intangible Assets, Net	38.0	134.5
Take-or-pay contracts | Fresh Start Reporting
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	$ 0.0	$ 0.0