Retirement Plans and Postretirement Benefit Plans Postretirement Benefit Plans (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018		Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Employer contributions		$ 15.0		$ 20.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Settlement		26.1		15.3	$ 8.6
Net periodic benefit cost		35.4		32.9	22.5
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Level 3 Asset Value, Beginning of Period		40.2
Interest income		0.7
Dividend income		0.5
Unrealized gains		(0.5)
Realized losses		(0.2)
Administrative expenses and charges		(0.1)
Claims paid		(4.6)
Level 3 Asset Value, End of Period		36.0		40.2
Postretirement Plan, Expected Future Benefit Payments, Next Twelve Months		11.6
Postretirement Plan, Expected Future Benefit Payments, Year Two		11.6
Postretirement Plan, Expected Future Benefit Payments, Year Three		11.6
Postretirement Plan, Expected Future Benefit Payments in Year Four		11.6
Postretirement Plan, Expected Future Benefit Payments, Year Five		10.2
Postretirement Plan, Expected Future Benefit Payments, Five Fiscal Years Thereafter		46.7
Postemployment Benefits Liability		1.9		2.5
Cash [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		0.5
Fair Value of Plan Assets, Ending				0.5
Fair Value, Inputs, Level 1 [Member] | Cash [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		0.5
Fair Value of Plan Assets, Ending				0.5
Fair Value, Inputs, Level 3 [Member] | Cash [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		0.0
Fair Value of Plan Assets, Ending				0.0
Pension Plan [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation, Beginning		687.5		672.2
Service cost		14.9		15.5	15.8
Interest cost		23.8		26.2	25.5
Plan settlements		(73.7)		(50.2)
Actuarial gains (losses)		(22.0)		38.6
Benefit Obligation, Ending		615.9		687.5	672.2
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		635.3		595.9
Actual return on plans' assets		(39.2)		84.4
Employer contributions		15.0		20.0
Plan settlements		(73.7)		(50.2)
Fair Value of Plan Assets, Ending		522.8		635.3	595.9
Funded Status of Plan		(93.1)		(52.2)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		14.9		15.5	15.8
Interest cost		23.8		26.2	25.5
Expected return on plan assets		(44.1)		(42.6)	(41.5)
Amortization of unrecognized prior service cost		0.0		(0.1)	(0.1)	[1]
Settlement		25.1		15.3	0.0
Net periodic benefit cost		35.9		31.7	16.2
Amount paid by unconsolidated affiliates		2.5		4.3	5.1
Net Periodic Benefit Cost, Net of Unconsolidated Affiliates	[2]	33.4		27.4	11.1
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Capitalized Portion of Net Periodic Benefit Cost		$ 3.8		$ 4.4	$ 4.0
Assumptions Used Calculating Benefit Obligation, Discount Rate		4.20%		3.60%	4.00%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate		3.73%		4.00%	4.00%
Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets		7.50%		7.50%	7.50%
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase		4.20%		4.20%	4.20%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase		4.20%		4.20%	4.20%
Defined Benefit Plan, Benefit Obligation, Benefits Paid		$ (14.6)		$ (14.8)
Pension Plan [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning					$ 0.0
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation, Beginning		149.4		215.9
Service cost		0.3		0.6	0.8
Interest cost		5.4		7.2	9.5
Plan settlements		0.0		(28.1)
Actuarial gains (losses)		(9.6)		5.6
Benefit Obligation, Ending		135.8		149.4	215.9
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		50.2		53.1
Actual return on plans' assets		(0.6)		2.8
Employer contributions		5.4		34.6
Plan settlements		0.0		(28.1)
Fair Value of Plan Assets, Ending		45.3		50.2	53.1
Funded Status of Plan		(90.5)		(99.2)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		0.3		0.6	0.8
Interest cost		5.4		7.2	9.5
Expected return on plan assets		(2.0)		(2.2)	(2.3)
Amortization of unrecognized prior service cost		(8.4)		(3.5)	(8.8)	[1]
Settlement		0.0		0.6	0.0
Net periodic benefit cost		(0.9)		4.7	1.8
Amount paid by unconsolidated affiliates		(0.5)		0.3	0.2
Net Periodic Benefit Cost, Net of Unconsolidated Affiliates	[2]	(0.4)		4.4	1.6
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Capitalized Portion of Net Periodic Benefit Cost		$ 0.2		$ 1.2	$ 0.8
Assumptions Used Calculating Benefit Obligation, Discount Rate		4.30%		3.70%	4.20%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate		3.70%		4.20%	4.25%
Health Care Cost Trend Rate Assumed for Next Fiscal Year		7.25%
Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets		4.00%		4.00%	4.00%
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase					4.20%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase				4.20%	4.20%
Defined Benefit Plan, Benefit Obligation, Benefits Paid		$ (13.5)		$ (15.7)
Other Postretirement Benefit Plans, Defined Benefit [Member] | Group Retiree Medical Insurance Contract [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		40.2
Fair Value of Plan Assets, Ending		36.0		40.2
Other Postretirement Benefit Plans, Defined Benefit [Member] | U.S. Equity Mutual Funds Investment [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		9.5
Fair Value of Plan Assets, Ending		8.9		9.5
Other Postretirement Benefit Plans, Defined Benefit [Member] | Cash [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Ending		0.9
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		10.0
Fair Value of Plan Assets, Ending		9.8		10.0
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 1 [Member] | Group Retiree Medical Insurance Contract [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		0.0
Fair Value of Plan Assets, Ending		0.0		0.0
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 1 [Member] | U.S. Equity Mutual Funds Investment [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		9.5
Fair Value of Plan Assets, Ending		8.9		9.5
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 1 [Member] | Cash [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Ending		0.9
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		0.0
Fair Value of Plan Assets, Ending		0.0		0.0
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		40.2
Fair Value of Plan Assets, Ending		36.0		40.2
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 3 [Member] | Group Retiree Medical Insurance Contract [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		40.2
Fair Value of Plan Assets, Ending		36.0		40.2
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 3 [Member] | U.S. Equity Mutual Funds Investment [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		0.0
Fair Value of Plan Assets, Ending		0.0		0.0
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 3 [Member] | Cash [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Ending		0.0
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 1, 2 and 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		50.2
Fair Value of Plan Assets, Ending		45.8		50.2
OKLAHOMA | Other Postretirement Benefit Plans, Defined Benefit [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Additional Postretirement Medical Expense to Meet State Requirements		4.4		6.2	$ 7.9
Equity Funds [Member] | Pension Plan [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning	[3]	29.9
Fair Value of Plan Assets, Ending	[3]	19.7	[4]	29.9
Equity Funds [Member] | Pension Plan [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning	[3]	0.0
Fair Value of Plan Assets, Ending	[3]	0.0		0.0
Equity Funds [Member] | Pension Plan [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning	[3]	0.0
Fair Value of Plan Assets, Ending	[3]	$ 0.0		$ 0.0

[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 $35.4 million, $32.9 million and $22.5 million of net periodic benefit cost recognized in 2018, 2017 and 2016, respectively, OG&E recognized the following: •a change in pension expense in 2018, 2017 and 2016 of $(14.1) million, $(2.3) million and $9.9 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 asset or liability (see Note 1); •an increase in postretirement medical expense in 2018, 2017 and 2016 of $4.4 million, $6.2 million and $7.9 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 asset or liability (see Note 1); and •a deferral of pension expense in 2018, 2017 and 2016 of $2.1 million, $1.1 million and $0.1 million related to the Arkansas jurisdictional portion of the pension settlement charge of $26.1 million, $15.3 million and $8.6 million, respectively, which are included in the Arkansas deferred pension expense regulatory asset (see Note 1).

[3] This category represents units of participation in a commingled fund that primarily invested in stocks of international companies and emerging markets.

[4] GAAP allows the measurement of certain investments that do not have a readily determinable fair value at the net asset value. These investments do not consider the observability of inputs; therefore, they are not included within the fair value hierarchy.