Supplemental Cash Flow Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Supplemental Cash Flow Information (Textuals)
Interest paid, net of amounts capitalized			$ 127.9	$ 118.0
Net refunds from income taxes			7.6
Income taxes paid, net of refunds				21.0
Accounts payable related to capital expenditures	10.2	18.8	10.2	18.8
Amortization of deferred revenue			28.8	27.5
Income (Loss) from Equity Method Investments	17.2	16.1	33.9	31.7
Distributions from transmission affiliate			26.8	25.8
Transmission Affiliate [Member]
Supplemental Cash Flow Information (Textuals)
Income (Loss) from Equity Method Investments	$ 17.3	$ 16.2	$ 33.9	$ 31.8