Regulated Operations (Details) - USD ($) $ in Millions		Mar. 31, 2021	Dec. 31, 2020
Schedule of Regulatory Assets and Liabilities [Line Items]
Fuel clause under recoveries		$ 91.8	$ 0.0
Current Regulatory Assets		110.5	19.8
Non-Current Regulatory Assets		1,269.7	415.6
Current Regulatory Liabilities		22.5	35.1
Fuel clause over recoveries		15.5	28.6
Non-Current Regulatory Liabilities		1,176.9	1,188.9
Oklahoma Demand Program Over Recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[1]	5.5	1.5
Other (B)
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[1]	1.5	5.0
Non-Current Regulatory Liabilities		4.4	4.7
Income taxes recoverable from customers, net
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		858.2	867.4
Accrued removal obligations, net
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		314.3	316.8
Production Tax Credit Rider [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets	[2]	5.2	4.4
Benefit obligations regulatory asset
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		141.2	164.9
Oklahoma deferred storm expenses
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		184.0	158.8
Smart Grid
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		9.5	11.2
Dry Scrubber Regulatory Asset [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		19.5	19.7
Unamortized loss on reacquired debt
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		9.5	9.7
Deferred Pension Expenses [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		11.2	9.3
Other Regulatory Asset [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets	[2]	5.1	8.4
Non-Current Regulatory Assets		10.9	11.1
Pension tracker
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		39.2	18.1
SPP Cost Tracker Rider Under Recovery
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets	[2]	8.4	7.0
COVID-19 Deferred Expenses
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		7.9	6.4
Extreme Cold Weather Costs
Schedule of Regulatory Assets and Liabilities [Line Items]
Fuel clause under recoveries		102.4
Non-Current Regulatory Assets		829.4	0.0
Frontier Plant
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		$ 7.4	$ 6.4

[1] Included in Other Current Liabilities in the balance sheets.    

[2] Included in Other Current Assets in the balance sheets.