Rate Matters and Regulation Rate Matters and Regulation (Details)	9 Months Ended
	Sep. 30, 2020 USD ($)
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 5,200,000
Estimated Refund to SPP	13,000,000.0
Investment in Grid Enhancement	800,000,000.0
Arkansas Environmental Refund	5,300,000
Plant Investments for Customers	28,000,000.0
Minimum [Member]
Revenue Requirement from OCC	13,000,000.0
Maximum [Member]
Revenue Requirement from OCC	20,000,000.0
OCC Favor to Coop Ruling [Member]
Plant Investments for Customers	11,700,000
OCC Favor to Coop Ruling [Member] | Minimum [Member]
Revenue Requirement from OCC	1,100,000
OCC Favor to Coop Ruling [Member] | Maximum [Member]
Revenue Requirement from OCC	1,800,000
Impact to Company [Member]
Estimated Refund to SPP	5,000,000.0
Customer Impact [Member]
Estimated Refund to SPP	$ 8,000,000.0