Regulated Operations (Details) - USD ($) $ in Millions		Jun. 30, 2019	Dec. 31, 2018
Schedule of Regulatory Assets and Liabilities [Line Items]
Fuel clause under recoveries		$ 30.7	$ 2.0
Current Regulatory Assets		48.1	18.5
Non-Current Regulatory Assets		290.0	285.8
Current Regulatory Liabilities		36.3	36.6
Customer Refund Liability, Current		0.1	0.3
Non-Current Regulatory Liabilities		1,243.0	1,270.7
Reserve for Tax Refund [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[1]	17.7	15.4
SPP Cost Tracker Rider Over Recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[1]	8.0	16.8
Oklahoma Demand Program Over Recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[1]	6.2	0.0
Transmission Cost Recovery Rider [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[1]	1.1	2.7
Other (B)
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[1]	3.2	1.4
Non-Current Regulatory Liabilities		6.3	6.8
Income taxes recoverable from customers, net
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		921.6	937.1
Accrued removal obligations, net
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		312.9	308.1
Pension tracker
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		2.2	18.7
Cogeneration Credit Rider Over Credit [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets	[2]	8.2	0.0
Oklahoma demand program rider under recovery (A)
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets	[2]	0.0	6.4
Production Tax Credit Rider [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets	[2]	4.1	6.9
Benefit obligations regulatory asset
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		172.3	188.2
Deferred storm expenses
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		43.6	36.5
Smart Grid
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		22.1	25.6
Dry Scrubber Regulatory Asset [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		21.0	4.5
Unamortized loss on reacquired debt
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		11.0	11.4
Deferred Pension Expenses [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		7.7	6.8
Other Regulatory Asset [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets	[2]	5.1	3.2
Non-Current Regulatory Assets		$ 12.3	$ 12.8

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

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