Investments (Summary Financial Information) (Details) (American Transmission Company LLC (ATC) [Member], USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2013	Mar. 31, 2012
American Transmission Company LLC (ATC) [Member]
Schedule of Equity Method Investments [Line Items]
Operating revenues	$ 151.7	$ 147.7
Operating income	82.0	78.1
Net income	$ 60.5	$ 58.1

The amount of gross profit (loss) reported by an equity method investment of the entity.

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+ Details

- Definition

The amount of net income (loss) reported by an equity method investment of the entity.

+ References

+ Details