Schedule I - Condensed Financial Information of Registrant - Balance Sheets (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Assets
Cash and cash equivalents	$ 209.9	$ 379.7	$ 234.2	$ 248.4
Operating assets and facilities, net	5,688.0us-gaap_PropertyPlantAndEquipmentNet	5,070.3us-gaap_PropertyPlantAndEquipmentNet
Investments in affiliated companies	357.7	354.3	502.0
Other assets	229.0us-gaap_OtherAssets	225.5us-gaap_OtherAssets
Total Assets	6,937.5us-gaap_Assets	6,549.6us-gaap_Assets	6,055.4us-gaap_Assets
Liabilities and Shareholders’ Equity
Accounts payable and accrued expenses	165.9	159.6
Debt	4,274.2us-gaap_DebtAndCapitalLeaseObligations	3,871.0us-gaap_DebtAndCapitalLeaseObligations
Other liabilities	246.1us-gaap_OtherLiabilities	230.6us-gaap_OtherLiabilities
Total Liabilities	5,623.5us-gaap_Liabilities	5,152.6us-gaap_Liabilities
Total Shareholders’ Equity	1,314.0us-gaap_StockholdersEquity	1,397.0us-gaap_StockholdersEquity	1,244.2us-gaap_StockholdersEquity
Total Liabilities and Shareholders’ Equity	6,937.5	6,549.6
Parent Company [Member]
Assets
Cash and cash equivalents	162.3	294.6	183.5	208.4
Operating assets and facilities, net	3,397.0	2,724.3
Investments in affiliated companies	2,199.5	2,197.9
Other assets	396.0	428.8
Total Assets	6,154.8	5,645.6
Liabilities and Shareholders’ Equity
Accounts payable and accrued expenses	54.9	79.6
Debt	3,821.1	3,249.2
Other liabilities	964.8	919.8
Total Liabilities	4,840.8	4,248.6
Total Shareholders’ Equity	1,314.0	1,397.0
Total Liabilities and Shareholders’ Equity	$ 6,154.8	$ 5,645.6

Sum of the carrying values as of the balance sheet date of obligations incurred through that date, including liabilities incurred and payable to vendors for goods and services received, taxes, interest, rent and utilities, compensation costs, payroll taxes and fringe benefits (other than pension and postretirement obligations), contractual rights and obligations, and statutory obligations.

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