Leases (Details) - USD ($) $ in Millions		3 Months Ended		12 Months Ended
		Mar. 31, 2019		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2019
Lessee, Lease, Description [Line Items]
Transitional Right-of-Use Asset Obtained in Exchange for Operating Lease Liability		$ 32.3
Lessee, Operating Lease, Contingent Liability		6.8		$ 16.2
Operating Leases, Future Minimum Payments, Due in Two Years		2.9
Operating Leases, Future Minimum Payments, Due in Three Years		2.9
Lessee, Operating Lease, Liability, Payments, Due Year Two		5.3
Lessee, Operating Lease, Liability, Payments, Due Next Twelve Months		1.9
Operating Lease, Cost		1.2
Cash Paid on Operating Leases		3.0
Right-of-Use Asset Obtained in Exchange for Finance Lease Liability	[1]	10.7
Operating Lease, Right-of-Use Asset		$ 42.1	[2]				$ 32.3
Operating Lease, Weighted Average Remaining Lease Term		13 years 10 months 24 days
Operating Lease, Weighted Average Discount Rate, Percent		3.90%
Lessee, Operating Lease, Liability, Payments, Due Year Three		$ 5.3
Lessee, Operating Lease, Liability, Payments, Due Year Four		5.2
Lessee, Operating Lease, Liability, Payments, Due Year Five		5.1
Lessee, Operating Lease, Liability, Payments, Due after Year Five		37.8
Lessee, Operating Lease, Liability, Payments, Due		60.6
Unrecorded Unconditional Purchase Obligation, Imputed Interest		13.8
Operating Lease, Liability		46.8	[3]				$ 36.9
Operating Leases, Future Minimum Payments, Due in Four Years		2.9
Operating Leases, Future Minimum Payments Due, Next Twelve Months		21.1
Operating Leases, Future Minimum Payments, Due in Five Years		2.9
Operating Leases, Future Minimum Payments, Due Thereafter		37.6
Operating Leases, Future Minimum Payments Due		70.3
Operating Leases, Rent Expense, Net				4.1	$ 5.4	$ 8.5
OG&E Railcar Lease Agreement [Member]
Lessee, Lease, Description [Line Items]
Operating Leases, Future Minimum Payments, Due in Two Years	[4]			0.0
Operating Leases, Future Minimum Payments, Due in Three Years	[4]			0.0
Operating Leases, Future Minimum Payments, Due in Four Years	[4]			0.0
Operating Leases, Future Minimum Payments Due, Next Twelve Months	[4]			18.6
Operating Leases, Future Minimum Payments, Due in Five Years	[4]			0.0
Operating Leases, Future Minimum Payments, Due Thereafter	[4]			0.0
Operating Leases, Future Minimum Payments Due	[4]	18.6
OG&E Wind Farm Land Lease Agreements [Member]
Lessee, Lease, Description [Line Items]
Operating Leases, Future Minimum Payments, Due in Two Years				2.9
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, Next Twelve Months				2.5
Operating Leases, Future Minimum Payments, Due in Five Years				2.9
Operating Leases, Future Minimum Payments, Due Thereafter				$ 37.6
Operating Leases, Future Minimum Payments Due		$ 51.7

[1] Three Months Ended March 31 (Dollars in millions)2019LEASE COST Operating lease cost$1.2 OTHER INFORMATIONCash paid for amounts included in the measurement of lease liabilities: Operating cash flows for operating leases$3.0Right-of-use assets obtained in exchange for new operating lease liabilities$10.7Right-of-use assets at period end (A)$42.1Operating lease liabilities at period end (B)$46.8Operating lease weighted-average remaining lease term (in years)13.9Operating lease weighted-average discount rate3.9%

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

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

[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.