Retirement Plans and Postretirement Benefit Plans (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Other Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		$ 0.0	$ 0.0	$ 0.1	$ 0.1
Interest cost		0.0	0.0	0.1	0.1
Expected return on plan assets		0.0	0.0	0.0	0.0
Amortization of net loss		0.1	0.1	0.3	0.3
Settlement cost		0.5	0.3	0.5	0.3
Net periodic benefit cost		0.6	0.4	1.0	0.8
Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		2.3	2.3	6.8	6.7
Interest cost		3.2	3.5	10.0	11.7
Expected return on plan assets		(7.0)	(7.2)	(20.9)	(20.7)
Amortization of net loss		3.0	3.3	9.0	9.7
Settlement cost		10.3	1.7	10.3	12.9
Net periodic benefit cost		11.8	3.6	15.2	20.3
Capitalized Portion of Net Periodic Benefit Cost		0.8	0.7	2.3	2.2
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		0.0	0.0	0.1	0.1
Interest cost		0.8	1.1	2.4	3.2
Expected return on plan assets		(0.4)	(0.4)	(1.2)	(1.3)
Amortization of net loss		0.5	0.6	1.6	1.7
Amortization of unrecognized prior service cost	[1]	(1.5)	(1.6)	(4.6)	(4.6)
Capitalized Portion of Net Periodic Benefit Cost		0.0	0.0	0.1	0.1
Postretirement Benefit Costs
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Net periodic benefit cost		(0.6)	(0.3)	(1.7)	(0.9)
OKLAHOMA | Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Settlement cost	[2],[3]	10.8	2.3	11.1	14.0
Additional Pension Expense to Meet State Requirements	[2],[3]	2.4	1.5	5.7	(0.2)
OKLAHOMA | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Additional Pension Expense to Meet State Requirements	[4],[5]	0.4	0.1	1.2	0.9
ARKANSAS | Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Settlement cost	[2],[3]	1.0	0.2	1.0	1.3
OGE Energy [Member] | Other Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Amount Attributable to Unconsolidated Affiliates		0.2	0.2	0.8	0.4
OGE Energy [Member] | Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Amount Attributable to Unconsolidated Affiliates		1.5	0.9	2.5	3.5
OGE Energy [Member] | Postretirement Benefit Costs
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Net periodic benefit cost		(0.2)	(0.2)	(0.6)	(0.5)
Og and E [Member] | Other Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Net periodic benefit cost		0.8	0.6	1.8	1.2
Og and E [Member] | Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Net periodic benefit cost		13.3	4.5	17.7	23.8
Og and E [Member] | Postretirement Benefit Costs
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Net periodic benefit cost		$ (0.8)	$ (0.5)	$ (2.3)	$ (1.4)

[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] Included in the pension regulatory asset or liability in each jurisdiction, as indicated in the regulatory assets and liabilities table in Note 1.

[3] Includes a portion of OGE Energy's pension expense.

[4] Included in the Pension tracker, as presented in the regulatory assets and liabilities table in Note 1.

[5] Includes a portion of OGE Energy's pension expense.