Regulated Operations (Details) - USD ($) $ in Millions		Mar. 31, 2022	Dec. 31, 2021
Schedule of Regulatory Assets and Liabilities [Line Items]
Fuel clause under recoveries		$ 182.3	$ 151.9
Current Regulatory Assets		203.4	182.6
Non-Current Regulatory Assets		1,256.7	1,230.8
Current Regulatory Liabilities		1.6	2.5
Non-Current Regulatory Liabilities		1,223.3	1,231.1
Other (B)
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[1]	1.6	2.5
Non-Current Regulatory Liabilities		3.3	3.6
Income taxes recoverable from customers, net
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		927.3	930.7
Accrued removal obligations, net
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		292.7	296.8
Winter Storm Uri costs | OKLAHOMA
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		749.2	747.9
Winter Storm Uri costs | ARKANSAS
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		86.3	88.9
Benefit obligations regulatory asset
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		104.4	109.2
Oklahoma deferred storm expenses
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		204.6	172.8
Smart Grid
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		2.2	3.9
Dry Scrubber Regulatory Asset [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		18.7	18.9
Unamortized loss on reacquired debt
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		8.7	8.9
Deferred Pension Expenses [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		12.3	12.1
Other Regulatory Asset [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets	[2]	8.3	9.7
Non-Current Regulatory Assets		10.4	10.4
Pension tracker
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		45.4	42.9
SPP Cost Tracker Rider Under Recovery
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets	[2]	9.4	9.3
COVID-19 Deferred Expenses
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		8.1	8.2
Oklahoma Energy Efficiency Rider Under Recovery
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets	[2]	3.4	11.7
Deferred Plant Expense
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		$ 6.4	$ 6.7

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

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