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]
Net periodic benefit cost (B)		$ 2.2	$ 4.2
Other Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		0.0	0.0
Interest cost		0.0	0.0
Expected return on plan assets		0.0	0.0
Amortization of net loss		0.1	0.1
Amortization of unrecognized prior service cost	[1]	0.0	0.0
Net periodic benefit cost (B)		0.1	0.1
Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		2.7	2.7
Interest cost		4.4	4.8
Expected return on plan assets		(8.5)	(8.3)
Amortization of net loss		3.0	3.0
Amortization of unrecognized prior service cost	[1]	0.0	0.0
Net periodic benefit cost (B)		1.6	2.2
Capitalized Portion of Net Periodic Benefit Cost		0.8	0.8
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		0.1	0.1
Interest cost		1.0	1.7
Expected return on plan assets		(0.5)	(0.5)
Amortization of net loss		1.0	0.6
Amortization of unrecognized prior service cost	[1]	(1.5)	0.0
Net periodic benefit cost (B)		0.1	1.9
Capitalized Portion of Net Periodic Benefit Cost		0.0	0.6
OKLAHOMA
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Additional Pension Expense to Meet State Requirements		4.0	2.9
OKLAHOMA | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Additional Pension Expense to Meet State Requirements		2.1	1.1
OGE Energy [Member] | Other Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Net periodic benefit cost (B)		0.1	0.0
OGE Energy [Member] | Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Net periodic benefit cost (B)		0.5	0.0
OGE Energy [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Net periodic benefit cost (B)		(0.2)	0.0
Og and E [Member] | Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Net periodic benefit cost (B)	[2]	2.1
Electric Utility [Member] | Other Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Net periodic benefit cost (B)	[2]	0.2	0.1
Electric Utility [Member] | Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Net periodic benefit cost (B)	[2]		2.2
Electric Utility [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Net periodic benefit cost (B)	[2]	$ (0.1)	$ 1.9

[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 $2.2 million and $4.2 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).