Regulated Operations (Details) - USD ($) $ in Millions		Jun. 30, 2021	Dec. 31, 2020
Schedule of Regulatory Assets and Liabilities [Line Items]
Fuel clause under recoveries		$ 0.4	$ 0.0
Current Regulatory Assets		21.8	19.8
Non-Current Regulatory Assets		1,278.3	415.6
Current Regulatory Liabilities		10.0	35.1
Fuel clause over recoveries		4.7	28.6
Non-Current Regulatory Liabilities		1,259.7	1,188.9
Oklahoma Demand Program Over Recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[1]	4.3	1.5
Other (B)
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[1]	1.0	5.0
Non-Current Regulatory Liabilities		4.1	4.7
Income taxes recoverable from customers, net
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		947.5	867.4
Accrued removal obligations, net
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		308.1	316.8
Benefit obligations regulatory asset
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		135.6	164.9
Oklahoma deferred storm expenses
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		181.8	158.8
Smart Grid
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		7.6	11.2
Dry Scrubber Regulatory Asset [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		19.3	19.7
Unamortized loss on reacquired debt
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		9.3	9.7
Deferred Pension Expenses [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		11.6	9.3
Other Regulatory Asset [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets	[2]	6.4	8.4
Non-Current Regulatory Assets		10.8	11.1
Pension tracker
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		40.6	18.1
SPP Cost Tracker Rider Under Recovery
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets	[2]	10.9	7.0
COVID-19 Deferred Expenses
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		7.1	6.4
Extreme Cold Weather Costs | OKLAHOMA
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		754.8	0.0
Extreme Cold Weather Costs | ARKANSAS
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		92.3	0.0
Frontier Plant
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		7.5	6.4
Generation Capacity Replacement
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets	[2]	$ 4.1	$ 4.4

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

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