Retirement Plans and Postretirement Benefit Plans (Details) - USD ($) $ in Millions		3 Months Ended				9 Months Ended
		Sep. 30, 2015		Sep. 30, 2014		Sep. 30, 2015		Sep. 30, 2014
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Net periodic benefit cost (net of unconsolidated affiliates)		$ 19.7		$ 3.3		$ 26.1		$ 9.5
Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost		4.2		3.9		12.1		11.5
Interest cost		6.6		7.0		19.6		21.1
Expected return on plan assets		(11.0)		(11.3)		(34.5)		(34.0)
Amortization of net loss		3.8		3.6		13.5		10.7
Amortization of unrecognized prior service cost	[1]	0.1		0.4		0.3		1.3
Settlement		16.2		0.0		16.2		0.0
Total net periodic benefit cost		19.9		3.6		27.2		10.6
Less: Amount paid by unconsolidated affiliates		1.0		0.9		3.1		2.6
Net periodic benefit cost (net of unconsolidated affiliates)		18.9	[2]	2.7	[2]	24.1	[3]	8.0	[3]
Capitalized Portion of Net Periodic Benefit Cost		2.6		0.9		4.6		2.6
Other Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost		0.4		0.3		1.0		0.8
Interest cost		0.2		0.2		0.5		0.5
Expected return on plan assets		0.0		0.0		0.0		0.0
Amortization of net loss		0.2		0.1		0.5		0.2
Amortization of unrecognized prior service cost	[1]	0.0		0.0		0.1		0.1
Settlement		0.0		0.0		0.0		0.0
Total net periodic benefit cost		0.8		0.6		2.1		1.6
Less: Amount paid by unconsolidated affiliates		0.0		0.0		0.1		0.1
Net periodic benefit cost (net of unconsolidated affiliates)		0.8	[2]	0.6	[2]	2.0	[3]	1.5	[3]
Postretirement Benefit Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost		0.4		0.7		1.2		2.3
Interest cost		2.6		2.8		7.7		8.5
Expected return on plan assets		(0.6)		(0.6)		(1.8)		(1.8)
Amortization of net loss		3.5		3.1		10.4		9.3
Amortization of unrecognized prior service cost	[1]	(4.1)		(4.1)		(12.4)		(12.4)
Total net periodic benefit cost		1.8		1.9		5.1		5.9
Less: Amount paid by unconsolidated affiliates		0.4		0.3		1.0		1.0
Net periodic benefit cost (net of unconsolidated affiliates)		1.4	[4]	1.6	[4]	4.1	[5]	4.9	[5]
Capitalized Portion of Net Periodic Benefit Cost		0.5		0.5		1.4		1.5
OKLAHOMA
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Additional Pension Expense to Meet State Requirements		2.7		2.8		8.0		8.4
OKLAHOMA | Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Settlement		7.4				7.4
OKLAHOMA | Postretirement Benefit Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Additional Pension Expense to Meet State Requirements		1.4		$ 1.3		4.3		$ 3.9
ARKANSAS | Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Settlement		$ 1.4				$ 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 $19.7 million and $3.3 million of net periodic benefit cost recognized during the three months ended September 30, 2015 and 2014, respectively, OG&E recognized the following:•an increase in pension expense during the three months ended September 30, 2015 and 2014 of $2.7 million and $2.8 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 $26.1 million and $9.5 million of net periodic benefit cost recognized during the nine months ended September 30, 2015 and 2014, respectively, OG&E recognized the following: •an increase in pension expense during the nine months ended September 30, 2015 and 2014 of $8.0 million and $8.4 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);

[4] In addition to the $1.4 million and $1.6 million of net periodic benefit cost recognized during the three months ended September 30, 2015 and 2014, respectively, OG&E recognized an increase in postretirement medical expense during the three months ended September 30, 2015 and 2014 of $1.4 million and $1.3 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).

[5] In addition to the $4.1 million and $4.9 million of net periodic benefit cost recognized during the nine months ended September 30, 2015 and 2014, respectively, OG&E recognized an increase in postretirement medical expense during the nine months ended September 30, 2015 and 2014 of $4.3 million and $3.9 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).