Retirement Plans and Postretirement Benefit Plans (Details) - USD ($) $ in Millions		3 Months Ended				6 Months Ended				12 Months Ended
		Jun. 30, 2018		Jun. 30, 2017		Jun. 30, 2018		Jun. 30, 2017		Dec. 31, 2013
Other Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		$ 0.1	[1]	$ 0.0	[1]	$ 0.1	[2]	$ 0.0	[2]
Interest cost		0.1	[1]	0.1	[1]	0.1	[2]	0.1	[2]
Expected return on plan assets		0.0	[1]	0.0	[1]	0.0	[2]	0.0	[2]
Amortization of net loss		0.2	[1]	0.1	[1]	0.3	[2]	0.2	[2]
Net periodic benefit cost		0.4	[1]	0.2	[1]	0.5	[2]	0.3	[2]
Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		2.3	[1]	2.4	[1]	5.0	[2]	5.1	[2]
Interest cost		4.3	[1]	4.9	[1]	8.7	[2]	9.7	[2]
Expected return on plan assets		(8.4)	[1]	(8.1)	[1]	(16.9)	[2]	(16.4)	[2]
Amortization of net loss		3.1	[1]	3.5	[1]	6.1	[2]	6.5	[2]
Settlement										$ (22.4)
Net periodic benefit cost		1.3	[1]	2.7	[1]	2.9	[2]	4.9	[2]
Capitalized Portion of Net Periodic Benefit Cost		0.8		1.0		1.6		1.8
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		0.0	[3]	0.1	[3]	0.1	[4]	0.2	[4]
Interest cost		1.1	[3]	1.6	[3]	2.1	[4]	3.3	[4]
Expected return on plan assets		(0.4)	[3]	(0.5)	[3]	(0.9)	[4]	(1.0)	[4]
Amortization of net loss		0.9	[3]	0.2	[3]	1.9	[4]	0.8	[4]
Amortization of unrecognized prior service cost	[5]	(1.6)	[3]	0.0	[3]	(3.1)	[4]	0.0	[4]
Net periodic benefit cost		0.0	[3]	1.4	[3]	0.1	[4]	3.3	[4]
Capitalized Portion of Net Periodic Benefit Cost		0.1		0.5		0.1		1.1
OKLAHOMA
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Additional Pension Expense to Meet State Requirements		3.8		2.9		7.8		5.8
OKLAHOMA | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Additional Pension Expense to Meet State Requirements		2.2		1.0		4.3		2.1
ARKANSAS | Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Settlement				(2.3)				(2.3)
OGE Energy [Member] | Other Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Net periodic benefit cost		0.2	[1]	0.0	[1]	0.3	[2]	0.0	[2]
OGE Energy [Member] | Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Net periodic benefit cost		0.5	[1]	0.0	[1]	1.0	[2]	0.0	[2]
OGE Energy [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Net periodic benefit cost		(0.1)	[3]	0.0	[3]	(0.3)	[4]	0.0	[4]
Og and E [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Net periodic benefit cost	[1]	2.4		2.9		4.7		5.2
Og and E [Member] | Other Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Net periodic benefit cost		0.6	[1]	0.2	[1]	0.8	[2]	0.3	[2]
Og and E [Member] | Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Net periodic benefit cost		1.8	[1]	2.7	[1]	3.9	[2]	4.9	[2]
Og and E [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Net periodic benefit cost		$ (0.1)	[3]	$ 1.4	[3]	$ (0.2)	[4]	$ 3.3	[4]

[1] In addition to the $2.4 million and $2.9 million of net periodic benefit cost recognized during the three months ended June 30, 2018 and 2017, respectively, OG&E recognized the following:•an increase in pension expense during the three months ended June 30, 2018 and 2017 of $3.8 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•a deferral of pension expense during the three months ended June 30, 2017 of $2.3 million related to the Arkansas jurisdictional portion of the pension settlement charge of $22.4 million in 2013.

[2] In addition to the $4.7 million and $5.2 million of net periodic benefit cost recognized during the six months ended June 30, 2018 and 2017, respectively, OG&E recognized the following:•an increase in pension expense during the six months ended June 30, 2018 and 2017 of $7.8 million and $5.8 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•a deferral of pension expense during the six months ended June 30, 2017 of $2.3 million related to the Arkansas jurisdictional portion of the pension settlement charge of $22.4 million in 2013.

[3] In addition to the $0.1 million of net periodic benefit income and $1.4 million of net periodic benefit cost recognized during the three months ended June 30, 2018 and 2017, respectively, OG&E recognized an increase in postretirement medical expense in the three months ended June 30, 2018 and 2017 of $2.2 million and $1.0 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).

[4] In addition to the $0.2 million of net periodic benefit income and $3.3 million of net periodic benefit cost recognized during the six months ended June 30, 2018 and 2017, respectively, OG&E recognized an increase in postretirement medical expense in the six months ended June 30, 2018 and 2017 of $4.3 million and $2.1 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).

[5] 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.