Intangible Contract Assets and Liabilities - Balances and Balance Sheet Classifications (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Fresh-Start Adjustment [Line Items]
Assets	$ 51.9	$ 70.9
Liabilities	(71.1)	(90.0)
Finite-Lived Liabilities, Net	(19.2)	(19.1)
Investments and other assets
Fresh-Start Adjustment [Line Items]
Assets	51.9	70.9
Liabilities	0.0	0.0
Finite-Lived Assets, Net	51.9	70.9
Accounts payable and accrued expenses
Fresh-Start Adjustment [Line Items]
Assets	0.0	0.0
Liabilities	(11.2)	(20.3)
Finite-Lived Liabilities, Net	(11.2)	(20.3)
Other noncurrent liabilities
Fresh-Start Adjustment [Line Items]
Assets	0.0	0.0
Liabilities	(59.9)	(69.7)
Finite-Lived Liabilities, Net	(59.9)	(69.7)
Coal supply agreements
Fresh-Start Adjustment [Line Items]
Liabilities	(25.5)	(32.9)
Take-or-pay contracts
Fresh-Start Adjustment [Line Items]
Liabilities	(45.6)	(57.1)
Finite-Lived Liabilities, Net	(45.6)	(57.1)
Coal supply agreements
Fresh-Start Adjustment [Line Items]
Assets	51.9	70.9
Finite-Lived Assets, Net	26.4	38.0
Take-or-pay contracts
Fresh-Start Adjustment [Line Items]
Assets	$ 0.0	$ 0.0