Investments (Summary Financial Information) (Details) (American Transmission Company LLC (ATC) [Member], USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
American Transmission Company LLC (ATC) [Member]
Schedule of Equity Method Investments [Line Items]
Operating revenues	$ 152.2	$ 152.1	$ 303.9	$ 299.8
Operating income	82.2	80.4	164.2	158.5
Net income	$ 61.3	$ 59.3	$ 121.8	$ 117.4