Condensed Financial Information of Registrant (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Assets
Cash and Cash Equivalents	$ 248.4	$ 78.5	$ 41.7	$ 102.2
Operating assets and facilities, net	4,359.3	4,133.8
Investments in Affiliated Companies	513.8	486.1	452.2
Other Assets	180.1	187.5
Total Assets	5,857.5	5,442.4	5,206.4
Liabilities and Shareholders' Equity
Accounts Payable and Accrued Expenses	135.6	114.6
Debt	3,547.1	3,176.5
Other Liabilities	281.6	287.0
Total Liabilities	4,730.2	4,328.7
Total Shareholders' Equity	1,127.3	1,113.7	1,102.6
Total Liabilities and Shareholders' Equity	5,857.5	5,442.4
Parent Company [Member]
Assets
Cash and Cash Equivalents	208.4	48.9	28.1	64.0
Operating assets and facilities, net	2,355.9	2,154.0
Investments in Affiliated Companies	2,010.9	1,668.1
Other Assets	478.1	438.9
Total Assets	5,053.3	4,309.9
Liabilities and Shareholders' Equity
Accounts Payable and Accrued Expenses	56.1	48.4
Debt	2,986.6	2,571.8
Other Liabilities	883.3	576.0
Total Liabilities	3,926.0	3,196.2
Total Shareholders' Equity	1,127.3	1,113.7
Total Liabilities and Shareholders' Equity	$ 5,053.3	$ 4,309.9