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Summary of Significant Accounting Policies Asset Retirement Obligations (Details) (USD $)
In Millions, unless otherwise specified
6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Balance at January 1 $ 53.6 $ 24.8
Liabilities settled 0 0
Accretion expense 1.1 0.9
Revisions in estimated cash flows 0 26.7 [1]
Balance at June 30 $ 54.7 $ 52.4
[1] Due to changes to OG&E's asset retirement obligations related to its wind farms as a result of changes in the assumption related to the timing of removal used in the valuation of the asset retirement obligations.