Summary of Significant Accounting Policies Regulated Operations (Details) - USD ($) $ in Millions		Dec. 31, 2019	Dec. 31, 2018
Schedule of Regulatory Assets and Liabilities [Line Items]
Fuel clause under recoveries		$ 39.5	$ 2.0
Fuel clause over recoveries		4.8	0.3
Regulatory Assets, Current		48.7	18.5
Regulatory Assets, Noncurrent		306.0	285.8
Regulatory Liability, Current		29.0	36.6
Regulatory Liability, Noncurrent		1,223.5	1,270.7
Other Regulatory Liabilities [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	[1]	6.9	1.4
Regulatory Liability, Noncurrent		5.8	6.8
Accrued removal obligations [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Noncurrent		318.5	308.1
Pension tracker [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Noncurrent		0.0	18.7
Oklahoma demand program rider under recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	[2]	0.0	6.4
Other Regulatory Assets [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	[2]	7.5	3.2
Regulatory Assets, Noncurrent		13.4	12.8
Benefit Obligations [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		167.2	188.2
Smart Grid [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		18.4	25.6
Deferred storm expenses [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		65.5	36.5
Unamortized loss on reacquired debt [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		$ 10.6	$ 11.4

[1] Included in Other Current Liabilities in the Balance Sheets.

[2] Included in Other Current Assets in the Balance Sheets.