Condensed Financial Information of Registrant - Balance Sheets (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Assets
Cash and cash equivalents	$ 379.7	$ 234.2	$ 248.4	$ 78.5
Operating assets and facilities, net	5,070.3	4,654.4
Investments in affiliated companies	354.3	502.0	513.8
Other assets	225.5	186.7
Total Assets	6,549.6	6,055.4	5,857.5
Liabilities and Shareholders’ Equity
Accounts payable and accrued expenses	159.6	177.4
Debt	3,871.0	3,567.9
Other liabilities	230.6	282.9
Total Liabilities	5,152.6	4,811.2
Total Shareholders’ Equity	1,397.0	1,244.2	1,127.3
Total Liabilities and Shareholders’ Equity	6,549.6	6,055.4
Parent Company [Member]
Assets
Cash and cash equivalents	294.6	183.5	208.4	48.9
Operating assets and facilities, net	2,724.3	2,501.7
Investments in affiliated companies	2,197.9	2,043.8
Other assets	428.8	440.0
Total Assets	5,645.6	5,169.0
Liabilities and Shareholders’ Equity
Accounts payable and accrued expenses	79.6	105.0
Debt	3,249.2	2,761.1
Other liabilities	919.8	1,058.7
Total Liabilities	4,248.6	3,924.8
Total Shareholders’ Equity	1,397.0	1,244.2
Total Liabilities and Shareholders’ Equity	$ 5,645.6	$ 5,169.0