Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current:
Federal		$ (0.6)	$ (14.8)	$ (14.0)
State and local		0.9	1.0	0.5
Foreign		1.6	0.0	0.0
Total current		1.9	(13.8)	(13.5)
Investment tax credits		(0.1)	(0.1)	(0.1)
Deferred:
Federal		(10.2)	47.1	73.2
State and local		5.7	2.3	5.7
Foreign		(1.0)	0.4	0.0
Total deferred		(5.5)	49.8	78.9
Valuation allowance		13.1	(9.2)	(3.6)
Total		$ 9.4	$ 26.7	$ 61.7
Effective Income Tax Rate Reconciliation, Percent [Abstract]
U.S. federal statutory rate		21.00%	35.00%	35.00%
State and local income taxes, net of federal income tax		1.40%	0.70%	0.20%
Change in valuation allowance, net of federal income tax		(21.80%)	(9.10%)	(1.40%)
State net operating loss adjustments		(10.10%)	2.00%	0.90%
Federal rate change		0.00%	3.50%	0.00%
Transaction costs		(3.10%)	5.50%	0.00%
Non-Deductible Meals and Entertainment		(1.80%)	1.30%	0.40%
Unrecognized tax benefit changes		0.00%	1.40%	2.30%
Other differences, net		(1.10%)	(0.20%)	0.10%
Effective tax rate		(15.50%)	40.10%	37.50%
Income tax expense (benefit)		$ 9.4	$ 26.7	$ 61.7
Accumulated other comprehensive income (loss)		1.3	(28.3)	43.8
Excess tax benefits on stock option exercises		$ 0.0	0.0	0.1
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards, Expiration Date		Dec. 31, 2023
Deferred Tax Assets, Valuation Allowance	$ 45.5	$ 58.2	45.5
Deferred tax assets:
Net operating loss carryforwards	88.9	184.0	88.9
Pension and postretirement benefits	46.1	55.0	46.1
Employee benefits	7.9	10.3	7.9
Interest limitation	0.0	15.3	0.0
Texas Margin Credit	10.5	12.2	10.5
Other	9.3	25.6	9.3
Total deferred tax assets	162.7	302.4	162.7
Valuation allowance	(45.5)	(58.2)	(45.5)
Total deferred tax assets, net of valuation allowance	117.2	244.2	117.2
Deferred tax liabilities:
Property, plant and equipment	115.9	196.1	115.9
Other	0.3	12.0	0.3
Total deferred tax liabilities	116.2	208.1	116.2
Net deferred tax assets	1.0	36.1	1.0
Deferred Tax Assets, Operating Loss Carryforwards, Components [Abstract]
Federal tax operating loss carryforwards		592.6
Federal tax operating loss carryforwards, deferred tax asset value		124.4
Deferred Tax Assets, Operating Loss Carryforwards, Foreign		1.3
Deferred Tax Assets, Operating Loss Carryforwards, State and Local Tax		58.3
Federal tax operating loss carryforwards, to be expired 2023		125.6
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	22.0	21.9	22.0
Income Tax Uncertainties [Abstract]
Balance, beginning of year		22.2	31.4	27.6
Change in tax positions for the current year		0.0	1.0	1.2
Unrecognized Tax Benefits, Increase Resulting from Prior Period Tax Positions			0.3	2.6
Unrecognized Tax Benefits, Decrease Resulting from Prior Period Tax Positions		(0.2)
Change related to decrease in federal tax rate		0.0	10.5	0.0
Balance, end of year	22.2	22.0	22.2	31.4
Tax Cuts and Jobs Act [Abstract]
Tax Cuts And Jobs Act Of 2017, Incomplete Accounting, Change In Tax Rate, Deferred Tax Asset, Provisional Income Tax Expense	6.8
Reduction in Uncertain Tax Positions, Included in Provisional Net Charge	$ 10.5
Continuing Operations [Member]
Deferred:
Total		9.4	26.7	61.7
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Income tax expense (benefit)		9.4	$ 26.7	$ 61.7
Interest Limitation [Member]
Operating Loss Carryforwards [Line Items]
Deferred Tax Assets, Valuation Allowance		15.3
Deferred tax assets:
Valuation allowance		$ (15.3)