Retirement Plans and Postretirement Benefit Plans (Details) - USD ($) $ in Millions		3 Months Ended				9 Months Ended				12 Months Ended
		Sep. 30, 2017		Sep. 30, 2016		Sep. 30, 2017		Sep. 30, 2016		Dec. 31, 2013
Defined Benefit Plan, Plan Assets, Contributions by Employer						$ 20.0
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment						42.9
Defined Benefit Plan, Plan Assets, Payment for Settlement						26.4
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Defined Benefit Plan, Net Periodic Benefit Cost, Net of Unconsolidated Affiliates		$ 3.2		$ 3.0		10.2		$ 17.8
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Settlement						27.9
Net Effect of Settlement on Other Comprehensive Income and Regulatory Assets						2.1
Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		3.9		4.0		11.6		11.9
Interest cost		6.5		6.4		19.6		19.1
Expected return on plan assets		10.7		10.4		32.0		31.1
Defined Benefit Plan, Amortization of Gain (Loss)		4.4		4.1		13.1		12.3
Amortization of unrecognized prior service cost	[1]	0.0		(0.1)		0.0		(0.1)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		4.1		4.0		12.3		12.1
Less: Amount paid by unconsolidated affiliates		1.2		1.3		2.9		3.8
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement		0.0		0.0		0.0		0.0		$ 22.4
Capitalized Portion of Net Periodic Benefit Cost		1.0		1.0		3.3		3.0
Defined Benefit Plan, Net Periodic Benefit Cost, Net of Unconsolidated Affiliates		2.9	[1]	2.7	[1]	9.4	[2]	8.3	[2]
Other Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		0.0		0.0		0.2		0.2
Interest cost		0.1		0.1		0.2		0.3
Expected return on plan assets		0.0		0.0		0.0		0.0
Defined Benefit Plan, Amortization of Gain (Loss)		0.1		0.2		0.3		0.5
Amortization of unrecognized prior service cost	[1]	0.1		0.1		0.1		0.1
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		0.3		0.4		0.8		9.8
Less: Amount paid by unconsolidated affiliates		0.0		0.1		0.0		0.3
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement		0.0		0.0		0.0		8.7
Defined Benefit Plan, Net Periodic Benefit Cost, Net of Unconsolidated Affiliates		0.3	[1]	0.3	[1]	0.8	[2]	9.5	[2]
Postretirement Benefit Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		0.1		0.2		0.5		0.6
Interest cost		1.6		2.4		5.9		7.1
Expected return on plan assets		0.6		0.6		1.7		1.7
Defined Benefit Plan, Amortization of Gain (Loss)		0.5		0.6		1.3		1.9
Amortization of unrecognized prior service cost	[3]	(1.4)		(2.1)		(1.4)		(6.5)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		0.8		0.5		5.2		1.4
Less: Amount paid by unconsolidated affiliates		(0.1)		0.0		0.5		0.1
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement		0.6		0.0		0.6		0.0
Capitalized Portion of Net Periodic Benefit Cost		0.1		0.2		1.3		0.6
Defined Benefit Plan, Net Periodic Benefit Cost, Net of Unconsolidated Affiliates		0.9	[4]	0.5	[4]	4.7	[5]	1.3	[5]
OKLAHOMA
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Additional Pension Expense to Meet State Requirements		2.7		2.4		8.5		7.4
OKLAHOMA | Postretirement Benefit Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Additional Pension Expense to Meet State Requirements		$ 1.9		$ 1.9		3.9		5.9
ARKANSAS | Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement						2.3
ARKANSAS | Other Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement								$ 0.1
OGE Energy [Member] | Other Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement						$ 0.6

[1] In addition to the $3.2 million and $3.0 million of net periodic benefit cost recognized during the three months ended September 30, 2017 and 2016, respectively, OG&E recognized an increase in pension expense during the three months ended September 30, 2017 and 2016 of $2.7 million and $2.4 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).

[2] In addition to the $10.2 million and $17.8 million of net periodic benefit cost recognized during the nine months ended September 30, 2017 and 2016, respectively, OG&E recognized the following: •an increase in pension expense during the nine months ended September 30, 2017 and 2016 of $8.5 million and $7.4 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);•a deferral of pension expense during the nine months ended September 30, 2017 of $2.3 million related to the Arkansas jurisdictional portion of the pension settlement charge of $22.4 million in 2013;•a deferral of pension expense during the nine months ended September 30, 2016 of $0.6 million related to the pension settlement charge of $8.7 million, in accordance with the Oklahoma pension tracker regulatory liability (see Note 1); and•a deferral of pension expense during the nine months ended September 30, 2016 of $0.1 million related to the Arkansas jurisdictional portion of the pension settlement charge of $8.7 million.

[3] 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.

[4] In addition to the $0.9 million and $0.5 million of net periodic benefit cost recognized during the three months ended September 30, 2017 and 2016, respectively, OG&E recognized an increase in postretirement medical expense of $1.9 million in both the three months ended September 30, 2017 and 2016 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] In addition to the $4.7 million and $1.3 million of net periodic benefit cost recognized during the nine months ended September 30, 2017 and 2016, respectively, OG&E recognized an increase in postretirement medical expense in the nine months ended September 30, 2017 and 2016 of $3.9 million and $5.9 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).