Summary of Significant Accounting Policies Depreciation and Amortization (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Provision for Depreciation Rate	3.00%	2.90%
Percent of Intangible Plant Balance Amortizable Over Next Three Years	92.40%
Percent of Intangible Plant Balance Amortizable Thereafter	7.60%
Redbud Plant [Member]
Amount of Acquisition Adjustments	148.3
Transmission Equipment [Member] | OG&E [Member]
Amount of Acquisition Adjustments	3.3	3.3