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Summary of Significant Accounting Policies Depreciation and Amortization (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Provision for Depreciation Rate 2.50% 3.00%
Projected provision for depreciation in next fiscal year 2.70%  
Percent Of Intangible Plant Balance Amortizable 98.60%  
Percent of Intangible Plant Balance Amortizable Thereafter 1.40%  
Transmission Equipment [Member] | OG&E [Member]    
Amount of Acquisition Adjustments $ 3.3  
Redbud Plant [Member]    
Amount of Acquisition Adjustments $ 148.3 $ 148.3