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Investments (Summary Financial Information) (Details) (American Transmission Company LLC (ATC) [Member], USD $)
In Millions
3 Months Ended 6 Months Ended
Jun. 30, 2011
Jun. 30, 2010
Jun. 30, 2011
Jun. 30, 2010
American Transmission Company LLC (ATC) [Member]
       
Operating revenues $ 138.2 $ 138.7 $ 277.8 $ 277.2
Operating income 75.2 76.1 151.7 151.8
Net income $ 55.7 $ 54.1 $ 109.9 $ 109.2