Rate Matters and Regulation Rate Matters and Regulation (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Estimated Refund to SPP	$ 13.0
Fuel clause under recoveries	91.8	$ 0.0
Plan Investments for Customers	28.0
Regulatory assets	1,269.7	415.6
Minimum [Member]
Lost Revenues on Plant Investments for Customers	20.1
Maximum [Member]
Lost Revenues on Plant Investments for Customers	27.8
In Favor Cooperatives
Plan Investments for Customers	11.7
In Favor Cooperatives | Minimum [Member]
Lost Revenues on Plant Investments for Customers	1.8
In Favor Cooperatives | Maximum [Member]
Lost Revenues on Plant Investments for Customers	2.8
Grid Enhancement
OCC Revenue Requirement	7.0
Extreme Cold Weather Costs
Fuel clause under recoveries	102.4
Regulatory assets	829.4	$ 0.0
ARKANSAS
Public Utilities, Requested Rate Increase (Decrease), Amount	6.7
Impact to Company [Member]
Estimated Refund to SPP	5.0
Customer Impact [Member]
Estimated Refund to SPP	$ 8.0