Summary of Significant Accounting Policies, Regulated Operations (Details) - USD ($) $ in Millions		Mar. 31, 2020	Dec. 31, 2019
Schedule of Regulatory Assets and Liabilities [Line Items]
Fuel clause under recoveries		$ 0.0	$ 39.5
Current Regulatory Assets		12.8	48.7
Non-Current Regulatory Assets		300.0	306.0
Current Regulatory Liabilities		30.4	29.0
Fuel clause over recoveries		15.6	4.8
Non-Current Regulatory Liabilities		1,216.1	1,223.5
Reserve for Tax Refund [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[1]	4.8	12.7
SPP Cost Tracker Rider Over Recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[1]	0.0	2.6
Oklahoma Demand Program Over Recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[1]	4.4	2.0
Other Regulatory Liability[Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[1]	5.6	6.9
Non-Current Regulatory Liabilities		5.5	5.8
Income taxes refundable to customers, net
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		890.9	899.2
Accrued removal obligations, net
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		319.7	318.5
Production Tax Credit Rider [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets	[2]	4.6	3.7
Benefit obligations regulatory asset
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		165.2	167.2
Deferred storm expenses
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		63.9	65.5
Dry Scrubber Regulatory Asset [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		20.4	20.6
Unamortized loss on reacquired debt
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		10.4	10.6
Deferred Pension Expenses [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		7.8	8.0
Other Regulatory Asset [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets	[2]	8.2	5.5
Non-Current Regulatory Assets		14.8	13.4
Pension tracker
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		0.9	2.3
Smart Grid [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		$ 16.6	$ 18.4

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

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