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Fair Value Measurements (Tables)
 9 Months Ended
Sep. 30, 2018
Fair Value Disclosures [Abstract]  
Schedule of Fair Value and Carrying Amount of PRM Financial Instruments [Table Text Block]
The following table summarizes the fair value and carrying amount of the Company's financial instruments at September 30, 2018 and December 31, 2017.
 
September 30,
December 31,
 
2018
2017
(In millions)
Carrying Amount 
Fair
Value
Carrying Amount 
 Fair
Value
Long-term Debt (including Long-term Debt due within one year):
 
 
 
 
Senior Notes
$
3,001.6

$
3,125.9

$
2,854.3

$
3,242.8

OG&E Industrial Authority Bonds
$
135.4

$
135.4

$
135.4

$
135.4

Tinker Debt
$
9.7

$
8.7

$
9.7

$
9.8