Leases (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019		Sep. 30, 2019		Dec. 31, 2018		Jan. 01, 2019
Operating Leased Assets [Line Items]
Lessee, Operating Lease, Contingent Liability	$ 6.8		$ 6.8		$ 16.2
Operating Leases, Rent Expense, Net					4.9
Operating lease cost	1.5		4.5
Cash Paid on Operating Leases			4.9
Right-of-Use Asset Obtained in Exchange for Operating Lease Liability			10.7
Operating Lease, Right-of-Use Asset	42.0	[1]	42.0	[1]			$ 34.5
Operating Lease, Liability	$ 46.8	[2]	$ 46.8	[2]			$ 39.1
Operating Lease, Weighted Average Remaining Lease Term	13 years 2 months 12 days		13 years 2 months 12 days
Operating Lease, Weighted Average Discount Rate, Percent	3.90%		3.90%
Lessee, Operating Lease, Liability, Payments, Remainder of Fiscal Year	$ 0.8		$ 0.8		22.1	[3],[4]
Lessee, Operating Lease, Liability, Payments, Due Year Two	6.2		6.2		3.9	[3],[4]
Lessee, Operating Lease, Liability, Payments, Due Year Three	5.9		5.9		3.5	[3],[4]
Lessee, Operating Lease, Liability, Payments, Due Year Four	5.2		5.2		2.9	[3],[4]
Lessee, Operating Lease, Liability, Payments, Due Year Five	5.1		5.1		2.9	[3],[4]
Lessee, Operating Lease, Liability, Payments, Due after Year Five	37.8		37.8		37.6	[3],[4]
Lessee, Operating Lease, Liability, Payments, Due	61.0		61.0		$ 72.9	[3],[4]
Unrecorded Unconditional Purchase Obligation, Imputed Interest	$ 14.2		$ 14.2

[1] Included in Property, Plant and Equipment in the 2019 Condensed Consolidated Balance Sheet.

[2] Included in Other Deferred Credits and Other Liabilities in the 2019 Condensed Consolidated Balance Sheet.

[3] Amounts included for comparability and accounted for in accordance with ASC 840, "Leases."

[4] At the end of the railcar lease term, which was February 1, 2019, OG&E had the option to either purchase the railcars at a stipulated fair market value or renew the lease. OG&E renewed the lease effective February 1, 2019. If OG&E chose not to purchase the railcars or renew the lease agreement and the actual fair value of the railcars was less than the stipulated fair market value, OG&E would have been responsible for the difference in those values up to a maximum of $16.2 million.