Regulated Operations (Details) - USD ($) $ in Millions		Sep. 30, 2021	Dec. 31, 2020
Schedule of Regulatory Assets and Liabilities [Line Items]
Fuel clause under recoveries		$ 98.2	$ 0.0
Current Regulatory Assets		115.3	19.8
Non-Current Regulatory Assets		1,264.9	415.6
Current Regulatory Liabilities		6.3	35.1
Fuel clause over recoveries		0.0	28.6
Non-Current Regulatory Liabilities		1,247.7	1,188.9
Oklahoma Demand Program Over Recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[1]	4.7	1.5
Other (B)
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[1]	1.6	5.0
Non-Current Regulatory Liabilities		3.8	4.7
Income taxes recoverable from customers, net
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		938.8	867.4
Accrued removal obligations, net
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		305.1	316.8
Benefit obligations regulatory asset
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		130.6	164.9
Oklahoma deferred storm expenses
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		173.8	158.8
Smart Grid
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		5.9	11.2
Dry Scrubber Regulatory Asset [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		19.1	19.7
Unamortized loss on reacquired debt
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		9.1	9.7
Deferred Pension Expenses [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		11.8	9.3
Other Regulatory Asset [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets	[2]	7.7	8.4
Non-Current Regulatory Assets		10.6	11.1
Pension tracker
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		41.6	18.1
SPP Cost Tracker Rider Under Recovery
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets	[2]	7.7	7.0
COVID-19 Deferred Expenses
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		8.0	6.4
Extreme Cold Weather Costs | OKLAHOMA
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		756.3	0.0
Extreme Cold Weather Costs | ARKANSAS
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		91.0	0.0
Frontier Plant
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		7.1	6.4
Generation Capacity Replacement
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets	[2]	$ 1.7	$ 4.4

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

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