Summary of Significant Accounting Policies Depreciation and Amortization (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Provision for Depreciation Rate	2.90%	2.90%	3.00%
Percent of Intangible Plant Balance Amortizable Over Next Three Years		48.30%
Percent of Intangible Plant Balance Amortizable Thereafter		51.70%
Redbud Plant [Member] [Member]
Public Utilities, Property, Plant and Equipment, Amount of Acquisition Adjustments		148.3
OG&E [Member]
Public Utilities, Property, Plant and Equipment, Amount of Acquisition Adjustments		3.3