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Summary of Significant Accounting Policies Fuel Inventories (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Fuel inventories $ 93.3 $ 100.7  
OG&E [Member]
     
Fuel inventories 76.8 76.9  
Enogex [Member]
     
Fuel inventories 16.5 23.7  
Inventory Write-down $ 5.5 $ 4.8 $ 0.3