Retirement Plans and Postretirement Benefit Plans Postretirement Benefit Plans (Details) - USD ($) $ in Millions		9 Months Ended	12 Months Ended
		Sep. 30, 2019	Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Level 3 Asset Value, Beginning of Period		$ 36.0	$ 36.0
Interest income			0.8
Dividend income			0.5
Unrealized gains			1.4
Administrative expenses and charges			(0.1)
Claims paid			(3.8)
Level 3 Asset Value, End of Period			34.8		$ 36.0
Postretirement Plan, Expected Future Benefit Payments in Year One			8.9
Postretirement Plan, Expected Future Benefit Payments in Year Two			8.9
Postretirement Plan, Expected Future Benefit Payments in Year Three			8.8
Postretirement Plan, Expected Future Benefit Payments in Year Four			7.4
Postretirement Plan, Expected Future Benefit Payments in Year Five			7.3
Postretirement Plan, Expected Future Benefit Payments Thereafter			32.4
Postemployment Benefits Liability			$ 1.7		1.7
Cash [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets					0.9
Cash [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets					0.9
Cash [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets					$ 0.0
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate			4.30%		3.70%		4.20%
Defined Benefit Plan, Health Care Cost Trend Rate Assumed, Next Fiscal Year					7.00%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets			4.00%		4.00%		4.00%
Plan settlements			$ 0.0		$ 0.0
Benefit Obligation			104.7		104.8		$ 115.8
Fair Value of Plan Assets			41.9		40.6		45.2
Funded Status of Plan			(62.8)		(64.2)
Plan investments attributable to affiliates			(5.0)		(5.2)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement			0.0		0.0		(0.4)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)			(1.2)		0.3		$ 3.8
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase							4.20%
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets			12.1		9.8
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets			0.0		0.0
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets			34.8		36.0
Other Postretirement Benefit Plans, Defined Benefit [Member] | Group Retiree Medical Insurance Contract [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets			34.8		36.0
Other Postretirement Benefit Plans, Defined Benefit [Member] | Group Retiree Medical Insurance Contract [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets			0.0		0.0
Other Postretirement Benefit Plans, Defined Benefit [Member] | Group Retiree Medical Insurance Contract [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets			34.8		36.0
Other Postretirement Benefit Plans, Defined Benefit [Member] | U.S. Equity Mutual Funds Investment [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets			10.9		8.9
Other Postretirement Benefit Plans, Defined Benefit [Member] | U.S. Equity Mutual Funds Investment [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets			10.9		8.9
Other Postretirement Benefit Plans, Defined Benefit [Member] | U.S. Equity Mutual Funds Investment [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets			0.0		$ 0.0
Other Postretirement Benefit Plans, Defined Benefit [Member] | Money market funds [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets			1.2
Other Postretirement Benefit Plans, Defined Benefit [Member] | Money market funds [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets			1.2
Other Postretirement Benefit Plans, Defined Benefit [Member] | Money market funds [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets			$ 0.0
Pension Plans [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate			3.63%		3.73%		4.00%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets			7.50%		7.50%		7.50%
Plan settlements			$ 45.6		$ 52.6
Benefit Obligation			462.0		453.6		$ 510.6
Fair Value of Plan Assets			399.1		387.6		477.2
Funded Status of Plan			(62.9)		(66.0)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement			(16.4)		(19.4)		(11.7)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)			$ 26.3		$ 25.8		$ 21.5
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase			4.20%		4.20%		4.20%
Pension Plans [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets			$ 0.0		$ 0.0
Pension Plans [Member] | Money market funds [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets			7.5		0.3
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Alternative Investment			7.5	[1]	0.3	[2]
Pension Plans [Member] | Money market funds [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets			0.0		0.0
Pension Plans [Member] | Money market funds [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets			0.0		0.0
Other Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Plan settlements			0.9		0.6
Benefit Obligation			6.1		6.0		$ 4.2
Fair Value of Plan Assets			0.0		0.0		0.0
Funded Status of Plan			(6.1)		(6.0)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement			(0.5)		(0.4)		0.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)			1.2		1.3		0.7
OG&E [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets			41.9		40.6
ARKANSAS | Pension Plans [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement	[3]		$ (1.7)		$ (2.1)		$ (1.1)

[1] (A)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.

[2] 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.

[3] Included in the pension regulatory asset or liability in each jurisdiction, as indicated in the regulatory assets and liabilities table in Note 1.