Regulated Operations (Details) - USD ($) $ in Millions		Sep. 30, 2018	Dec. 31, 2017
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets		$ 18.4	$ 40.8
Non-Current Regulatory Assets		256.2	283.0
Current Regulatory Liabilities		52.1	3.9
Customer Refund Liability, Current		39.2	1.7
Non-Current Regulatory Liabilities		1,291.0	1,283.4
Other (B)
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[1]	3.5	2.2
Non-Current Regulatory Liabilities		6.9	7.2
Income taxes recoverable from customers, net
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		943.9	955.5
Accrued removal obligations, net
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		308.2	288.4
Pension tracker
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		32.0	32.3
Oklahoma demand program rider under recovery (A)
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets	[2]	10.7	31.6
Production Tax Credit Rider [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets	[2]	6.1	0.0
SPP cost tracker under recovery (A)
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets	[2]	0.0	7.7
Current Regulatory Liabilities	[1]	9.4	0.0
Benefit obligations regulatory asset
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		160.8	177.2
Deferred storm expenses
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		37.4	42.2
Smart Grid
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		27.5	32.8
Unamortized loss on reacquired debt
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		11.6	12.3
Other Regulatory Asset [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets	[2]	1.6	1.5
Non-Current Regulatory Assets		$ 18.9	$ 18.5

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

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