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Retirement Plans and Postretirement Benefit Plans (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]    
Defined Benefit Plan, Net Periodic Benefit Cost, Net of Unconsolidated Affiliates $ 2.3 $ 5.5
Pension Plan [Member]    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]    
Service cost 4.1 4.2
Interest cost 5.9 6.5
Expected return on plan assets 11.3 10.7
Defined Benefit Plan, Amortization of Gain (Loss) 3.9 4.0
Amortization of unrecognized prior service cost [1] 0.0 0.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) 2.6 4.0
Less: Amount paid by unconsolidated affiliates 0.5 0.8
Capitalized Portion of Net Periodic Benefit Cost 1.0 1.0
Defined Benefit Plan, Net Periodic Benefit Cost, Net of Unconsolidated Affiliates [2] 2.1 3.2
Other Pension Plan [Member]    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]    
Service cost 0.1 0.1
Interest cost 0.1 0.1
Expected return on plan assets 0.0 0.0
Defined Benefit Plan, Amortization of Gain (Loss) 0.1 0.1
Amortization of unrecognized prior service cost [1] 0.0 0.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) 0.3 0.3
Less: Amount paid by unconsolidated affiliates 0.0 0.0
Defined Benefit Plan, Net Periodic Benefit Cost, Net of Unconsolidated Affiliates [2] 0.3 0.3
Postretirement Benefit Plan [Member]    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]    
Service cost 0.1 0.2
Interest cost 1.3 2.2
Expected return on plan assets 0.5 0.6
Defined Benefit Plan, Amortization of Gain (Loss) 1.0 0.6
Amortization of unrecognized prior service cost [1] (2.1) 0.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) (0.2) 2.4
Less: Amount paid by unconsolidated affiliates (0.1) 0.4
Capitalized Portion of Net Periodic Benefit Cost 0.0 0.7
Defined Benefit Plan, Net Periodic Benefit Cost, Net of Unconsolidated Affiliates [2] (0.1) 2.0
OKLAHOMA    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]    
Additional Pension Expense to Meet State Requirements 4.0 2.9
OKLAHOMA | Postretirement Benefit Plan [Member]    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]    
Additional Pension Expense to Meet State Requirements $ 2.1 $ 1.1
[1] Unamortized prior service cost is amortized on a straight-line basis over the average remaining service period to the first eligibility age of participants who are expected to receive a benefit and are active at the date of the plan amendment.
[2] In addition to the $2.3 million and $5.5 million of net periodic benefit cost recognized during the three months ended March 31, 2018 and 2017, respectively, OG&E recognized the following:•an increase in pension expense during the three months ended March 31, 2018 and 2017 of $4.0 million and $2.9 million, respectively, to maintain the allowable amount to be recovered for pension expense in the Oklahoma jurisdiction, which are included in the Pension tracker regulatory liability (see Note 1); and•an increase in postretirement medical expense in the three months ended March 31, 2018 and 2017 of $2.1 million and $1.1 million, respectively, to maintain the allowable amount to be recovered for postretirement medical expense in the Oklahoma jurisdiction, which are included in the Pension tracker regulatory liability (see Note 1).