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Commitments and Contingencies (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Loss Contingencies [Line Items]      
Operating Leases, Future Minimum Payments Due, Next Twelve Months $ 4.8    
Operating Leases, Future Minimum Payments, Due in Two Years 23.4    
Operating Leases, Future Minimum Payments, Due in Three Years 2.9    
Operating Leases, Future Minimum Payments, Due in Four Years 2.9    
Operating Leases, Future Minimum Payments, Due in Five Years 2.9    
Operating Leases, Future Minimum Payments, Due Thereafter 40.6    
Operating Leases, Future Minimum Payments Due 77.5    
Operating Leases, Rent Expense, Net 6.2 $ 9.3 $ 7.7
Purchase Obligation, Due in Next Twelve Months 402.8    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 244.4    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 173.2    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 170.9    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 168.5    
Long-term Purchase Commitment, Amount 1,159.8    
Utilities Operating Expense, Purchased Power under Long-term Contracts 61.1 61.0 56.6
OG&E Railcar Lease Agreement [Member]      
Loss Contingencies [Line Items]      
Operating Leases, Future Minimum Payments Due, Next Twelve Months 1.7    
Operating Leases, Future Minimum Payments, Due in Two Years 20.9    
Operating Leases, Future Minimum Payments, Due in Three Years 0.0    
Operating Leases, Future Minimum Payments, Due in Four Years 0.0    
Operating Leases, Future Minimum Payments, Due in Five Years 0.0    
Operating Leases, Future Minimum Payments, Due Thereafter 0.0    
Operating Leases, Future Minimum Payments Due 22.6    
OG&E Wind Farm Land Lease Agreements [Member]      
Loss Contingencies [Line Items]      
Operating Leases, Future Minimum Payments Due, Next Twelve Months 2.5    
Operating Leases, Future Minimum Payments, Due in Two Years 2.5    
Operating Leases, Future Minimum Payments, Due in Three Years 2.9    
Operating Leases, Future Minimum Payments, Due in Four Years 2.9    
Operating Leases, Future Minimum Payments, Due in Five Years 2.9    
Operating Leases, Future Minimum Payments, Due Thereafter 40.6    
Operating Leases, Future Minimum Payments Due 54.3    
OGE Energy Building Lease [Member]      
Loss Contingencies [Line Items]      
Operating Leases, Future Minimum Payments Due, Next Twelve Months 0.6    
Operating Leases, Future Minimum Payments, Due in Two Years 0.0    
Operating Leases, Future Minimum Payments, Due in Three Years 0.0    
Operating Leases, Future Minimum Payments, Due in Four Years 0.0    
Operating Leases, Future Minimum Payments, Due in Five Years 0.0    
Operating Leases, Future Minimum Payments, Due Thereafter 0.0    
Operating Leases, Future Minimum Payments Due 0.6    
OG&E cogeneration capacity and fixed operation and maintenance payments [Member]      
Loss Contingencies [Line Items]      
Purchase Obligation, Due in Next Twelve Months 72.8    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 65.3    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 53.2    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 49.5    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 45.4    
Long-term Purchase Commitment, Amount 286.2    
OG&E expected cogeneration energy payments [Member]      
Loss Contingencies [Line Items]      
Purchase Obligation, Due in Next Twelve Months 35.7    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 35.6    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 35.9    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 37.1    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 38.3    
Long-term Purchase Commitment, Amount 182.6    
OG&E minimum fuel purchase commitments [Member]      
Loss Contingencies [Line Items]      
Purchase Obligation, Due in Next Twelve Months 139.8    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 36.2    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 24.6    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 24.6    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 24.6    
Long-term Purchase Commitment, Amount 249.8    
OG&E expected wind purchase commitments [Member]      
Loss Contingencies [Line Items]      
Purchase Obligation, Due in Next Twelve Months 58.7    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 56.5    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 56.9    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 57.3    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 57.8    
Long-term Purchase Commitment, Amount 287.2    
OG&E long-term service agreement commitments [Member]      
Loss Contingencies [Line Items]      
Purchase Obligation, Due in Next Twelve Months 7.9    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 41.9    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 2.4    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 2.4    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 2.4    
Long-term Purchase Commitment, Amount 57.0    
Mustang Modernization [Member]      
Loss Contingencies [Line Items]      
Purchase Obligation, Due in Next Twelve Months 24.9    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 0.0    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 0.0    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 0.0    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 0.0    
Long-term Purchase Commitment, Amount 24.9    
Environmental compliance plan expenditures [Member]      
Loss Contingencies [Line Items]      
Purchase Obligation, Due in Next Twelve Months 63.0    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 8.9    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 0.2    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 0.0    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 0.0    
Long-term Purchase Commitment, Amount $ 72.1    
Public Utility Regulatory Policy Act of 1978 [Member]      
Loss Contingencies [Line Items]      
Percentage of output purchased 100.00%    
OG&E total cogeneration payments [Member]      
Loss Contingencies [Line Items]      
Long-term Purchase Commitment, Amount $ 115.2 124.8 124.0
OG&E capacity payments [Member]      
Loss Contingencies [Line Items]      
Long-term Purchase Commitment, Amount 63.0 66.3 69.5
CPV Keenan [Member]      
Loss Contingencies [Line Items]      
Utilities Operating Expense, Purchased Power under Long-term Contracts 29.0 29.2 26.7
Edison Mission Energy [Member]      
Loss Contingencies [Line Items]      
Utilities Operating Expense, Purchased Power under Long-term Contracts 22.1 21.1 19.7
FPL Energy [Member]      
Loss Contingencies [Line Items]      
Utilities Operating Expense, Purchased Power under Long-term Contracts 2.6 3.4 3.2
NextEra Energy [Member]      
Loss Contingencies [Line Items]      
Utilities Operating Expense, Purchased Power under Long-term Contracts $ 7.4 $ 7.3 $ 7.0
OG&E long-term service agreement commitments [Member] | McClain Plant [Member]      
Loss Contingencies [Line Items]      
Factored-Fired Hours 128,000    
Factored-Fired Starts 4,800    
OG&E long-term service agreement commitments [Member] | Redbud Plant [Member]      
Loss Contingencies [Line Items]      
Factored-Fired Hours 144,000    
Factored-Fired Starts 4,500    
Additional Factored-Fired Hours 24,000    
Maximum [Member]      
Loss Contingencies [Line Items]      
Loss Contingency, Range of Possible Loss, Maximum $ 18.2