Retirement Plans and Postretirement Benefit Plans (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Net periodic benefit cost	$ 1.6		$ 1.5
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost	2.7		2.6
Interest cost	4.8		5.4
Expected return on plan assets	(9.3)		(9.9)
Amortization of net loss	3.2		2.8
Amortization of unrecognized prior service cost	0.1	[1]	0.5	[1]
Net periodic benefit cost	1.5	[2]	1.4	[2]
Capitalized Portion of Net Periodic Benefit Cost	0.5		0.4
Other Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost	0		0
Interest cost	0.1		0
Expected return on plan assets	0		0
Amortization of net loss	0		0
Amortization of unrecognized prior service cost	0	[1]	0.1	[1]
Net periodic benefit cost	0.1	[2]	0.1	[2]
Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost	0.3		0.6
Interest cost	2.0		2.3
Expected return on plan assets	(0.5)		(0.6)
Amortization of net loss	2.9		2.7
Amortization of unrecognized prior service cost	(3.4)	[1]	(3.4)	[1]
Net periodic benefit cost	1.3	[3]	1.6	[3]
Capitalized Portion of Net Periodic Benefit Cost	0.4		0.5
OKLAHOMA
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Additional Pension Expense to Meet State Requirements	3.0		3.3
OKLAHOMA | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Additional Pension Expense to Meet State Requirements	$ 1.4		$ 1.2

[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 $1.6 million and $1.5 million of net periodic benefit cost recognized during the three months ended March 31, 2015 and 2014, respectively, OG&E recognized an increase in pension expense during the three months ended March 31, 2015 and 2014 of $3.0 million and $3.3 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).

[3] In addition to the $1.3 million and $1.6 million of net periodic benefit cost recognized during the three months ended March 31, 2015 and 2014, respectively, OG&E recognized an increase in postretirement medical expense during the three months ended March 31, 2015 and 2014 of $1.4 million and $1.2 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).