Retirement Plans and Postretirement Benefit Plans (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Net periodic benefit cost		$ 4.2	$ 3.1
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost		2.7	2.8
Interest cost		4.8	5.0
Expected return on plan assets		(8.3)	(8.4)
Amortization of net loss		3.0	3.2
Amortization of unrecognized prior service cost	[1]	0.0	0.0
Net periodic benefit cost	[2]	2.2	2.6
Capitalized Portion of Net Periodic Benefit Cost		0.8	0.9
Other Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost		0.0	0.0
Interest cost		0.0	0.1
Expected return on plan assets		0.0	0.0
Amortization of net loss		0.1	0.0
Amortization of unrecognized prior service cost	[1]	0.0	0.0
Net periodic benefit cost	[2]	0.1	0.1
Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost		0.1	0.1
Interest cost		1.7	1.8
Expected return on plan assets		(0.5)	(0.5)
Amortization of net loss		0.6	0.5
Amortization of unrecognized prior service cost	[1]	0.0	(1.5)
Net periodic benefit cost	[2]	1.9	0.4
Capitalized Portion of Net Periodic Benefit Cost		0.6	0.2
OKLAHOMA
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Additional Pension Expense to Meet State Requirements		2.9	2.3
OKLAHOMA | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Additional Pension Expense to Meet State Requirements		$ 1.1	$ 2.0

[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 $4.2 million and $3.1 million of net periodic benefit cost recognized during the three months ended March 31, 2017 and 2016, respectively, OG&E recognized the following: •an increase in pension expense during the three months ended March 31, 2017 and 2016 of $2.9 million and $2.3 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); and•an increase in postretirement medical expense in the three months ended March 31, 2017 and 2016 of $1.1 million and $2.0 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).