Condensed Financial Information of Registrant - Balance Sheets (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Assets
Cash and cash equivalents	$ 234.2	$ 248.4	$ 78.5	$ 41.7
Operating assets and facilities, net	4,654.4	4,359.3
Investments in affiliated companies	502.0	513.8	486.1
Other assets	186.7	180.1
Total Assets	6,055.4	5,857.5	5,442.4
Liabilities and Shareholders’ Equity
Accounts payable and accrued expenses	177.4	135.6
Debt	3,567.9	3,547.1
Other liabilities	282.9	281.6
Total Liabilities	4,811.2	4,730.2
Total Shareholders’ Equity	1,244.2	1,127.3	1,113.7
Total Liabilities and Shareholders’ Equity	6,055.4	5,857.5
Parent Company [Member]
Assets
Cash and cash equivalents	183.5	208.4	48.9	28.1
Operating assets and facilities, net	2,501.7	2,355.9
Investments in affiliated companies	2,043.8	2,010.9
Other assets	440.0	478.1
Total Assets	5,169.0	5,053.3
Liabilities and Shareholders’ Equity
Accounts payable and accrued expenses	105.0	56.1
Debt	2,761.1	2,986.6
Other liabilities	1,058.7	883.3
Total Liabilities	3,924.8	3,926.0
Total Shareholders’ Equity	1,244.2	1,127.3
Total Liabilities and Shareholders’ Equity	$ 5,169.0	$ 5,053.3