Retirement Plans and Postretirement Benefit Plans (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Net Periodic Benefit Cost		$ 3.1	$ 2.9
Restoration of Retirement Income Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost		0.0	0.0
Interest cost		0.1	0.1
Expected return on plan assets		0.0	0.0
Amortization of net loss		0.0	0.0
Amortization of unrecognized prior service cost	[1]	0.0	0.0
Net Periodic Benefit Cost	[2]	0.1	0.1
Pension Plans [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost		2.8	2.7
Interest cost		5.0	4.8
Expected return on plan assets		(8.4)	(9.3)
Amortization of net loss		3.2	3.2
Amortization of unrecognized prior service cost	[1]	0.0	0.1
Net Periodic Benefit Cost	[2]	2.6	1.5
Capitalized Portion of Net Periodic Benefit Cost		0.9	0.5
Pension Plans [Member] | OKLAHOMA
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Additional Pension Expense to Meet State Requirements		2.3	3.0
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost		0.1	0.3
Interest cost		1.8	2.0
Expected return on plan assets		(0.5)	(0.5)
Amortization of net loss		0.5	2.9
Amortization of unrecognized prior service cost	[1]	(1.5)	(3.4)
Net Periodic Benefit Cost	[2]	0.4	1.3
Capitalized Portion of Net Periodic Benefit Cost		0.2	0.4
Other Postretirement Benefit Plans, Defined Benefit [Member] | OKLAHOMA
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Additional Postretirement Medical Expense to Meet State Requirements		$ 2.0	$ 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 $3.1 million and $2.9 million of net periodic benefit cost recognized during the three months ended March 31, 2016 and 2015, respectively, OG&E recognized the following: •an increase in pension expense in the three months ended March 31, 2016 and 2015 of $2.3 million and $3.0 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, 2016 and 2015 of $2.0 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). Three Months Ended March 31,(In millions)20162015Capitalized portion of net periodic pension benefit cost$0.9$0.5Capitalized portion of net periodic postretirement benefit cost0.20.4