Income Taxes (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards, Expiration Date		Dec. 31, 2023
Deferred tax assets:
Net operating loss carryforwards		$ 286.5	$ 452.3
Pension and postretirement benefits		95.5	81.9
Equity method investment in CyrusOne		64.5	41.5
Deferred tax assets, deferred gain on sale of wireless spectrum licenses		42.2
Number of Operating Segments	4	3
Deferred Tax Assets, Deferred Gain on Sale Leaseback Transaction			0
Other		47.4	46.7
Total deferred tax assets		560.8	638.9
Valuation allowance		(64.4)	(68.3)
Total deferred tax assets, net of valuation allowance		496.4	570.6
Deferred tax liabilities:
Property, plant and equipment		121.9	171.8
Other		4.9	3.8
Total deferred tax liabilities		126.8	175.6
Net deferred tax assets		369.6	395.0
Deferred Tax Assets, Operating Loss Carryforwards, Components [Abstract]
Federal tax operating loss carryforwards		665.2
Federal tax operating loss carryforwards, deferred tax asset value		232.8
Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax		24.7	16.5
State tax credits		10.8
Deferred Tax Assets, Operating Loss Carryforwards, Foreign, State and Local Tax		53.7
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		0.40%	1.50%	3.90%
Change in valuation allowance, net of federal income tax		(3.00%)	(15.80%)	(2.30%)
State net operating loss adjustments		2.90%	2.70%	3.70%
Nondeductible interest expense		4.00%	(11.40%)	18.10%
Unrecognized tax benefit changes		2.10%	(2.10%)	2.20%
Nondeductible compensation		1.00%	(2.50%)	2.70%
Foreign		0.00%	(0.70%)	3.50%
Other differences, net		0.80%	(2.30%)	2.00%
Effective tax rate		43.20%	4.40%	68.80%
Current:
Federal		8.0	0	1.8
State and local		1.6	0	1.6
Total current		9.6	0	3.4
Investment tax credits		(0.2)	(0.2)	(0.3)
Deferred:
Federal		48.4	(13.0)	21.8
State and local		0.7	(3.7)	2.0
Foreign		0	0.3	(0.5)
Total deferred		49.1	(16.4)	23.3
Valuation allowance		(1.1)	14.1	(1.7)
Total		57.4	(2.5)	24.7
Income Tax Provision (Benefit) Charged to Continuing Operations, Accumulated Other Comprehensive Income (Loss) or Additional Paid-In Capital [Abstract]
Income tax expense (benefit)		57.4	(2.5)	24.7
Accumulated other comprehensive income (loss)		(22.4)	42.1	(0.4)
Excess tax benefits on stock option exercises		(0.1)	(0.5)	(2.4)
Income Tax Uncertainties [Abstract]
Unrecognized Tax Benefits that Would Impact Effective Tax Rate		26.3	23.5
Balance, beginning of year	22.8	24.1	22.8	21.8
Change in tax positions for the current year		3.0	1.3	1.4
Change in tax positions for prior years		0	0	(0.4)
Balance, end of year		$ 27.1	$ 24.1	$ 22.8