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Fair Value Measurements (Tables)
 12 Months Ended
Dec. 31, 2018
Fair Value Disclosures [Abstract]  
Fair Value, by Balance Sheet Grouping [Table Text Block]
The following table summarizes the fair value and carrying amount of the Company's financial instruments at December 31, 2018 and 2017.
 
2018
2017
December 31 (In millions)
Carrying Amount 
Fair
Value
Carrying Amount 
 Fair
Value
Long-term Debt (including Long-term Debt due within one year):
 
 
 
 
Senior Notes
$
3,001.9

$
3,178.2

$
2,854.3

$
3,242.8

OG&E Industrial Authority Bonds
$
135.4

$
135.4

$
135.4

$
135.4

Tinker Debt
$
9.6

$
8.7

$
9.7

$
9.8