Summary of Significant Accounting Policies Regulated Operations (Details) - USD ($) $ in Millions		Dec. 31, 2020	Dec. 31, 2019
Schedule of Regulatory Assets and Liabilities [Line Items]
Fuel clause under recoveries		$ 0.0	$ 39.5
Fuel clause over recoveries		28.6	4.8
Regulatory Assets, Current		19.8	48.7
Regulatory Assets, Noncurrent		415.6	306.0
Regulatory Liability, Current		35.1	29.0
Regulatory Liability, Noncurrent		$ 1,188.9	1,223.5
CenterPoint [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Percentage Share of Management Rights		50.00%
OGE Energy [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Percentage Share of Management Rights		50.00%
SPP Cost Tracker Rider Over Recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	[1]	$ 0.0	2.6
Reserve for Tax Refund [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	[1]	0.8	12.7
Income taxes recoverable from customers, net [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Noncurrent		867.4	899.2
Accrued removal obligations [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Noncurrent		316.8	318.5
Other Regulatory Liabilities [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	[1]	4.2	6.9
Regulatory Liability, Noncurrent		4.7	5.8
Oklahoma demand program rider over recovery
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	[1]	1.5	2.0
SPP cost tracker under recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	[2]	7.0	0.0
Benefit obligations regulatory asset [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		164.9	167.2
Deferred storm expenses [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		158.8	65.5
Smart Grid [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		11.2	18.4
Unamortized loss on reacquired debt [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		9.7	10.6
Deferred Pension Expenses [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		9.3	8.0
Dry Scrubber Regulatory Asset [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		19.7	20.6
Other Regulatory Assets [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	[2]	8.4	5.5
Regulatory Assets, Noncurrent		11.1	13.4
Pension tracker [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		18.1	2.3
Frontier Plant
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		6.4	0.0
COVID-19 Impact
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		6.4	0.0
Generation Capacity Replacement rider
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	[2]	$ 4.4	$ 3.7

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

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