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
SPP Cost Tracker Rider Over Recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	[1]	2.6	16.8
Reserve for Tax Refund [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	[1]	12.7	15.4
Transmission Cost Recovery Rider [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	[1]	0.0	2.7
Income taxes recoverable from customers, net [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Noncurrent		899.2	937.1
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
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
Oklahoma demand program rider over recovery
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	[1]	2.0	0.0
Production Tax Credit Rider [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	[2]	1.7	6.9
Oklahoma demand program rider under recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	[2]	0.0	6.4
Benefit obligations regulatory asset [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		167.2	188.2
Deferred storm expenses [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		65.5	36.5
Smart Grid [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		18.4	25.6
Unamortized loss on reacquired debt [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		10.6	11.4
Deferred Pension Expenses [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		8.0	6.8
Dry Scrubber Regulatory Asset [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		20.6	4.5
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
Pension tracker [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		$ 2.3	$ 0.0

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

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