Regional Transmission Organizations (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Regional Transmission Organizations Disclosure [Abstract]
Reduction to sales and purchased power expense from reporting RTO transactions from net basis reporting	$ 68.6	$ 91.1	$ 78.0

The transactions for whichan entity 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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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration