Summary of Significant Accounting Policies Regulated Operations (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2021	Dec. 02, 2021	Dec. 31, 2020
Schedule of Regulatory Assets and Liabilities [Line Items]
Fuel clause under recoveries		$ 151.9		$ 0.0
Fuel clause over recoveries		0.0		28.6
Regulatory Assets, Current		182.6		19.8
Regulatory Assets, Noncurrent		1,230.8		415.6
Regulatory Liability, Current		2.5		35.1
Regulatory Liability, Noncurrent		1,231.1		1,188.9
Prior Period Reclassification Adjustment		$ 23.1
Investment Owned, Balance, Shares		95,389,721	110,982,805
Income taxes recoverable from customers, net [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Noncurrent		$ 930.7		867.4
Accrued removal obligations [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Noncurrent		296.8		316.8
Other Regulatory Liabilities [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	[1]	2.5		6.5
Regulatory Liability, Noncurrent		3.6		4.7
Oklahoma demand program rider under recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	[2]	11.7		0.0
SPP cost tracker under recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	[2]	9.3		7.0
Benefit obligations regulatory asset [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		109.2		164.9
Deferred storm expenses [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		172.8		158.8
Smart Grid [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		3.9		11.2
Unamortized loss on reacquired debt [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		8.9		9.7
Deferred Pension Expenses [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		12.1		9.3
Dry Scrubber Regulatory Asset [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		18.9		19.7
Other Regulatory Assets [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	[2]	8.7		8.4
Regulatory Assets, Noncurrent		10.4		11.1
Pension tracker [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		42.9		18.1
Frontier Plant
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		6.7		6.4
COVID-19 Impact
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		8.2		6.4
Generation Capacity Replacement rider
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	[2]	1.0		4.4
Winter Storm Uri Costs | OKLAHOMA
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		747.9		0.0
Winter Storm Uri Costs | ARKANSAS
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		$ 88.9		$ 0.0

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

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