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 (net of unconsolidated affiliates)		$ 5.5	$ 4.0
Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost		4.2	4.4
Interest cost		6.5	6.6
Expected return on plan assets		(10.7)	(10.5)
Amortization of net loss		4.0	4.2
Amortization of unrecognized prior service cost	[1]	0.0	0.0
Total net periodic benefit cost		4.0	4.7
Less: Amount paid by unconsolidated affiliates		0.8	1.3
Net periodic benefit cost (net of unconsolidated affiliates)	[2]	3.2	3.4
Capitalized Portion of Net Periodic Benefit Cost		1.0	1.2
Other Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost		0.1	0.1
Interest cost		0.1	0.1
Expected return on plan assets		0.0	0.0
Amortization of net loss		0.1	0.2
Amortization of unrecognized prior service cost	[1]	0.0	0.0
Total net periodic benefit cost		0.3	0.4
Less: Amount paid by unconsolidated affiliates		0.0	0.0
Net periodic benefit cost (net of unconsolidated affiliates)	[2]	0.3	0.4
Postretirement Benefit Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost		0.2	0.3
Interest cost		2.2	2.3
Expected return on plan assets		(0.6)	(0.6)
Amortization of net loss		0.6	0.5
Amortization of unrecognized prior service cost		0.0	(2.2)
Total net periodic benefit cost		2.4	0.3
Less: Amount paid by unconsolidated affiliates		0.4	0.1
Net periodic benefit cost (net of unconsolidated affiliates)		2.0	0.2
Capitalized Portion of Net Periodic Benefit Cost		0.7	0.1
OKLAHOMA
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Additional Pension Expense to Meet State Requirements		2.9	2.3
OKLAHOMA | 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 $5.5 million and $4.0 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 for regulatory liability (see Note 1).