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Fair Value Measurements (Tables)
12 Months Ended
Dec. 31, 2020
Fair Value Disclosures [Abstract]  
Fair Value, by Balance Sheet Grouping [Table Text Block] The following table presents the carrying amount and fair value of the Registrants' financial instruments at December 31, 2020 and 2019, as well as the classification level within the fair value hierarchy.
 20202019
December 31 (In millions)
Carrying Amount Fair
Value
Carrying Amount  Fair
Value
Classification
Long-term Debt (including Long-term Debt due within one year):  
OG&E Senior Notes$3,349.6 $4,182.1 $3,050.3 $3,500.4 Level 2
OG&E Industrial Authority Bonds$135.4 $135.4 $135.4 $135.4 Level 2
Tinker Debt$9.4 $10.7 $9.5 $10.0 Level 3