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Fair Value Measurements (Tables)
 9 Months Ended
Sep. 30, 2021
Fair Value Disclosures [Abstract]  
Schedule of Fair Value and Carrying Amount of PRM Financial Instruments [Table Text Block] The following table presents the carrying amount and fair value of the Registrants' financial instruments at September 30, 2021 and December 31, 2020, as well as the classification level within the fair value hierarchy.
September 30,December 31,
 20212020
(In millions)Carrying Amount Fair
Value		Carrying Amount Fair
Value		Classification
Long-term Debt (including Long-term Debt due within one year):    
OGE Energy Senior Notes$499.9 $500.1 $— $— Level 2
OG&E Senior Notes$3,851.1 $4,503.4 $3,349.6 $4,182.1 Level 2
OG&E Industrial Authority Bonds$135.4 $135.4 $135.4 $135.4 Level 2
OG&E Tinker Debt$9.4 $10.0 $9.4 $10.7 Level 3