Retirement Plans and Postretirement Benefit Plans (Details) - USD ($) $ in Millions		1 Months Ended	3 Months Ended				9 Months Ended
		Jul. 31, 2016	Sep. 30, 2016		Sep. 30, 2015		Sep. 30, 2016		Sep. 30, 2015
Pension Contributions		$ 20.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Net periodic benefit cost			$ 2.4		$ 12.1		$ 7.2		$ 16.2
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost			2.6		2.7		7.7		7.7
Interest cost			4.8		5.1		14.4		14.9
Expected return on plan assets			(8.2)		(8.9)		(24.8)		(27.4)
Amortization of net loss			3.1		2.9		9.3		10.3
Amortization of unrecognized prior service cost	[1]		0.0		0.1		0.0		0.4
Settlement			0.0		10.0		0.0		10.0
Net periodic benefit cost			2.3	[2]	11.9	[2]	6.6	[3]	15.9	[3]
Capitalized Portion of Net Periodic Benefit Cost			0.8		0.6		2.2		1.9
Other Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost			0.0		0.1		0.0		0.1
Interest cost			0.1		0.1		0.1		0.1
Expected return on plan assets			0.0		0.0		0.0		0.0
Amortization of net loss			0.0		0.0		0.1		0.1
Amortization of unrecognized prior service cost	[1]		0.0		0.0		0.0		0.0
Settlement			0.0		0.0		0.4		0.0
Net periodic benefit cost			0.1	[2]	0.2	[2]	0.6	[3]	0.3	[3]
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements							(0.4)
Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost			0.1		0.3		0.4		0.8
Interest cost			1.8		1.9		5.5		5.9
Expected return on plan assets			(0.6)		(0.5)		(1.6)		(1.6)
Amortization of net loss			0.6		3.0		1.9		8.9
Amortization of unrecognized prior service cost	[1]		(1.5)		(3.4)		(4.6)		(10.2)
Net periodic benefit cost			0.4	[4]	1.3	[4]	1.6	[5]	3.8	[5]
Capitalized Portion of Net Periodic Benefit Cost			0.2		0.4		0.6		1.2
OKLAHOMA
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Additional Pension Expense to Meet State Requirements			2.4		(4.7)		6.7		0.6
OKLAHOMA | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Additional Pension Expense to Meet State Requirements			1.9		1.4		5.9		4.3
ARKANSAS | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Settlement			$ 0.0		$ 1.4		$ 0.1		$ 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] In addition to the $2.4 million and $12.1 million of net periodic benefit cost recognized during the three months ended September 30, 2016 and 2015, respectively, OG&E recognized the following: •an increase in pension expense during the three months ended September 30, 2016 of $2.4 million and a deferral of $4.7 million for the three months ended September 30, 2015, 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•during the three months ended September 30, 2016 there were no costs relating to the deferral of pension expense compared to $1.4 million for the three months ended September 30, 2015 related to the Arkansas jurisdictional portion of the pension settlement charge of $10.0 million during the three months ended September 30, 2015.

[3] In addition to the $7.2 million and $16.2 million of net periodic benefit cost recognized during the nine months ended September 30, 2016 and 2015, respectively, OG&E recognized the following: •an increase in pension expense during the nine months ended September 30, 2016 and 2015 of $6.7 million and $0.6 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•costs relating to the deferral of pension expense during the nine months ended September 30, 2016 and 2015 of $0.1 million and $1.4 million, respectively, related to the Arkansas jurisdictional portion of the pension settlement charge of $0.4 million and $10.0 million, respectively.

[4] In addition to the $0.4 million and $1.3 million of net periodic benefit cost recognized during the three months ended September 30, 2016 and 2015, respectively, OG&E recognized an increase in postretirement medical expense during the three months ended September 30, 2016 and 2015 of $1.9 million and $1.4 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] In addition to the $1.6 million and $3.8 million of net periodic benefit cost recognized during the nine months ended September 30, 2016 and 2015, respectively, OG&E recognized an increase in postretirement medical expense during the nine months ended September 30, 2016 and 2015 of $5.9 million and $4.3 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).