Rate Matters and Regulation Rate Matters and Regulation (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Estimated Refund to SPP	$ 13.0
Fuel clause under recoveries	98.2	$ 0.0
Plan Investments for Customers	28.0
Regulatory assets	1,264.9	415.6
Securitization Authorization Requested	760.0
Securitization Fuel Purchase Request	748.9
Securitization Fuel Cost Request Prudency	739.1
Grid Enhancement
Revenue Requirement	7.0
Minimum [Member]
Lost Revenues on Plant Investments for Customers	26.0
Maximum [Member]
Lost Revenues on Plant Investments for Customers	35.4
In Favor Cooperatives
Plan Investments for Customers	11.7
In Favor Cooperatives | Minimum [Member]
Lost Revenues on Plant Investments for Customers	2.5
In Favor Cooperatives | Maximum [Member]
Lost Revenues on Plant Investments for Customers	3.9
ARKANSAS
Public Utilities, Requested Rate Increase (Decrease), Amount	6.4
Public Utilities, Approved Rate Increase (Decrease), Amount	6.7
ARKANSAS | Extreme Cold Weather Costs
Regulatory assets	91.0	$ 0.0
Impact to Company [Member]
Estimated Refund to SPP	5.0
Customer Impact [Member]
Estimated Refund to SPP	$ 8.0