Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current:
Federal		$ (14.8)	$ (14.0)	$ 9.2
State and local		1.0	0.5	1.7
Total current		(13.8)	(13.5)	10.9
Investment tax credits		(0.1)	(0.1)	(0.2)
Deferred:
Federal		51.7	72.6	149.4
State and local		2.3	5.7	5.2
Total deferred		54.0	78.3	154.6
Valuation allowance		(9.2)	(3.6)	(5.5)
Total		$ 30.9	$ 61.1	$ 159.8
Effective Income Tax Rate Reconciliation, Percent [Abstract]
U.S. federal statutory rate		35.00%	35.00%	35.00%
State and local income taxes, net of federal income tax		0.70%	0.20%	0.70%
Change in valuation allowance, net of federal income tax		(9.10%)	(1.40%)	(0.80%)
State net operating loss adjustments		2.00%	0.90%	0.30%
Transaction Costs		5.50%	0.00%	0.00%
Unrecognized tax benefit changes		1.40%	2.30%	0.20%
Federal Rate Change		10.30%	0.00%	0.00%
Other differences, net		1.00%	0.50%	0.10%
Effective tax rate		46.80%	37.50%	35.50%
Income tax expense (benefit)		$ 30.9	$ 61.1	$ 159.8
Accumulated other comprehensive income (loss)		(28.3)	43.8	2.0
Excess tax benefits on stock option exercises		$ 0.0	0.1	(0.1)
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards, Expiration Date		Dec. 31, 2023
Deferred tax assets:
Net operating loss carryforwards	$ 88.9	$ 88.9	125.2
Pension and postretirement benefits	46.1	46.1	78.7
Employee benefits	7.9	7.9	12.2
Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax	1.5	1.5	17.4
Texas Margin Credit	10.5	10.5	10.7
Other	14.9	14.9	19.1
Total deferred tax assets	169.8	169.8	263.3
Valuation allowance	(45.5)	(45.5)	(54.4)
Total deferred tax assets, net of valuation allowance	124.3	124.3	208.9
Deferred tax liabilities:
Property, plant and equipment	115.9	115.9	135.0
Investment in CyrusOne	0.0	0.0	9.1
Other	0.3	0.3	0.3
Total deferred tax liabilities	116.2	116.2	144.4
Net deferred tax assets	8.1	8.1	64.5
Deferred Tax Assets, Operating Loss Carryforwards, Components [Abstract]
Federal tax operating loss carryforwards	187.5	187.5
Federal tax operating loss carryforwards, deferred tax asset value	39.3	39.3
Deferred Tax Assets, Operating Loss Carryforwards, Foreign	1.6	1.6
State tax credits	10.5	10.5
Deferred Tax Assets, Operating Loss Carryforwards, Foreign, State and Local Tax	48.0	48.0
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	22.0	22.0	31.0
Federal tax operating loss carryforwards, to be expired 2023	130.0	130.0
Income Tax Uncertainties [Abstract]
Balance, beginning of year		31.4	27.6	27.1
Change in tax positions for the current year		1.0	1.2	0.5
Change in tax positions for prior years		0.3	2.6	0.0
Change related to decrease in federal tax rate		(10.5)	0.0	0.0
Balance, end of year	22.2	22.2	31.4	27.6
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		30.9	61.1	159.8
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Income tax expense (benefit)		30.9	61.1	159.8
Discontinued Operations [Member]
Deferred:
Total		0.0	0.0	34.8
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Income tax expense (benefit)		$ 0.0	$ 0.0	$ 34.8