Supplemental Cash Flow Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Supplemental Cash Flow Information (Textuals)
Interest paid, net of amounts capitalized			$ 118.0	$ 125.1
Income taxes paid, net of refunds			21.0
Net refunds from income taxes				49.4
Accounts payable related to capital expenditures	18.8	51.2	18.8	51.2
Amortization of deferred revenue			(27.5)	(26.9)
Income (Loss) from Equity Method Investments	16.1	15.1	31.7	30.5
Distributions from transmission affiliate			25.8	24.6
Transmission Affiliate [Member]
Supplemental Cash Flow Information (Textuals)
Income (Loss) from Equity Method Investments	$ 16.2	$ 15.2	$ 31.8	$ 30.7