Summary of Significant Accounting Policies Regulated Operations (Details) - USD ($) $ in Millions		Dec. 31, 2018	Dec. 31, 2017
Schedule of Regulatory Assets and Liabilities [Line Items]
Fuel clause under recoveries		$ 2.0	$ 0.0
Fuel clause over recoveries		0.3	1.7
Regulatory Assets, Current		18.5	40.8
Regulatory Assets, Noncurrent		285.8	283.0
Regulatory Liability, Current		36.6	3.9
Regulatory Liability, Noncurrent		1,270.7	1,283.4
Other Regulatory Liabilities [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	[1]	1.4	2.0
Regulatory Liability, Noncurrent		6.8	7.2
Accrued removal obligations [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Noncurrent		308.1	288.4
Pension tracker [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Noncurrent		18.7	32.3
Oklahoma demand program rider under recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	[2]	6.4	31.6
SPP cost tracker rider under recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	[2]	0.0	7.7
Other Regulatory Assets [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	[2]	3.2	1.5
Regulatory Assets, Noncurrent		12.8	13.4
Benefit Obligations [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		188.2	177.2
Smart Grid [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		25.6	32.8
Deferred storm expenses [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		36.5	42.2
Unamortized loss on reacquired debt [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		$ 11.4	$ 12.3

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

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