Income Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Current:
Federal	$ (14.0)	$ 9.2	$ 9.3
State and local	0.5	1.7	1.9
Total current	(13.5)	10.9	11.2
Investment tax credits	(0.1)	(0.2)	(0.2)
Deferred:
Federal	72.6	149.4	69.6
State and local	5.7	5.2	1.9
Total deferred	78.3	154.6	71.5
Valuation allowance	(3.6)	(5.5)	(1.1)
Total	$ 61.1	$ 159.8	$ 81.4
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	0.20%	0.70%	0.80%
Change in valuation allowance, net of federal income tax	(1.40%)	(0.80%)	(2.00%)
State net operating loss adjustments	0.90%	0.30%	1.90%
Nondeductible interest expense	0.00%	0.00%	2.70%
Unrecognized tax benefit changes	2.30%	0.20%	1.40%
Nondeductible compensation	0.20%	0.10%	0.70%
Other differences, net	0.30%	0.00%	0.40%
Effective tax rate	37.50%	35.50%	40.90%
Income tax expense (benefit)	$ 61.1	$ 159.8	$ 81.4
Accumulated other comprehensive income (loss)	43.8	2.0	(22.4)
Excess tax benefits on stock option exercises	$ 0.1	(0.1)	(0.1)
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards, Expiration Date	Dec. 31, 2023
Deferred tax assets:
Net operating loss carryforwards	$ 125.2	142.0
Pension and postretirement benefits	78.7	89.1
Deferred Tax Assets, Investments	0.0	68.9
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Employee Benefits	12.2	15.2
Deferred tax asset, tax deferred expense, Texas Margin Tax	10.7	10.7
Other	19.1	17.9
Total deferred tax assets	263.3	376.5
Valuation allowance	(54.4)	(58.4)
Total deferred tax assets, net of valuation allowance	208.9	318.1
Deferred tax liabilities:
Property, plant and equipment	135.0	134.9
Deferred Tax Liabilities, Investments	9.1	0.0
Other	0.3	0.3
Total deferred tax liabilities	144.4	135.2
Net deferred tax assets	64.5	182.9
Deferred Tax Assets, Operating Loss Carryforwards, Components [Abstract]
Federal tax operating loss carryforwards	219.4
Federal tax operating loss carryforwards, deferred tax asset value	76.8
State tax credits	10.7
Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax	17.4	32.7
Deferred Tax Assets, Operating Loss Carryforwards, Foreign, State and Local Tax	48.2
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	31.0	27.3
Income Tax Uncertainties [Abstract]
Balance, beginning of year	27.6	27.1	24.1
Change in tax positions for the current year	1.2	0.5	3.0
Change in tax positions for prior years	2.6	0.0	0.0
Balance, end of year	31.4	27.6	27.1
Continuing Operations [Member]
Deferred:
Total	61.1	159.8	81.4
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Income tax expense (benefit)	61.1	159.8	81.4
Discontinued Operations [Member]
Deferred:
Total	0.0	34.8	(24.0)
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Income tax expense (benefit)	$ 0.0	$ 34.8	$ (24.0)