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 (net of unconsolidated affiliates) (B)	$ 3.1		$ 2.4
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost	4.5		4.3
Interest cost	6.4		7.0
Expected return on plan assets	(11.8)		(12.5)
Amortization of net loss	4.3		3.5
Amortization of unrecognized prior service cost	0.1	[1]	0.4	[1]
Defined Benefit Plan, Net Periodic Benefit Cost	3.5		2.7
Amount paid by unconsolidated affiliates	1.1		0.8
Net periodic benefit cost (net of unconsolidated affiliates) (B)	2.4	[2]	1.9	[2]
Capitalized Portion of Net Periodic Benefit Cost	0.8		0.7
Other Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost	0.4		0.3
Interest cost	0.2		0.2
Expected return on plan assets	0		0
Amortization of net loss	0.1		0
Amortization of unrecognized prior service cost	0	[1]	0	[1]
Defined Benefit Plan, Net Periodic Benefit Cost	0.7		0.5
Amount paid by unconsolidated affiliates	0		0
Net periodic benefit cost (net of unconsolidated affiliates) (B)	0.7	[2]	0.5	[2]
Postretirement Benefit Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost	0.5		0.9
Interest cost	2.6		2.8
Expected return on plan assets	(0.6)		(0.6)
Amortization of net loss	3.4		3.1
Amortization of unrecognized prior service cost	(4.1)	[1]	(4.1)	[1]
Defined Benefit Plan, Net Periodic Benefit Cost	1.8		2.1
Amount paid by unconsolidated affiliates	0.3		0.3
Net periodic benefit cost (net of unconsolidated affiliates) (B)	1.5	[3]	1.8	[3]
Capitalized Portion of Net Periodic Benefit Cost	0.5		0.6
OKLAHOMA
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Additional Pension Expense to Meet State Requirements	3.0		3.3
OKLAHOMA | 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 $3.1 million and $2.4 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 $1.5 million and $1.8 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).