Regional Transmission Organizations (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Regional Transmission Organizations [Abstract]
Reduction to sales and purchased power expense from reporting RTO transactions on a net basis	$ 75.9	$ 102.1	$ 94.9

For the MISO and PJM markets the transactions for which it buys and sells power within the same hour to meet electric energy delivery requirements are reported on a net basis. The treatment results in a reduction to sales to the market and purchased power expense.

No definition available.

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