Summary of Significant Accounting Policies Regulated Operations (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2016	Dec. 31, 2015
Schedule of Regulatory Assets and Liabilities [Line Items]
Fuel clause over recoveries		$ 63.2	$ 61.3
Regulatory Assets, Current		50.8	46.5
Regulatory Assets, Noncurrent		402.8	402.2
Regulatory Liability, Current		66.9	68.8
Regulatory Liability, Noncurrent		278.3	273.6
Prior Period Reclassification Adjustment		1.0
Other Regulatory Liabilities [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	[1]	3.7	7.5
Regulatory Liability, Noncurrent	[2]	1.0	1.0
Accrued removal obligations [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Noncurrent		255.3	254.9
Pension tracker [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Noncurrent		22.0	17.7
Oklahoma demand program rider under recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	[3]	37.4	36.6
Other Regulatory Assets [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	[3]	13.4	9.9
Regulatory Assets, Noncurrent		17.1	17.3
Benefit obligations regulatory asset [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		240.0	242.2
Income taxes recoverable from customers, net [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		56.8	56.7
Smart Grid [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		43.6	43.6
Deferred storm expenses [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		30.9	27.6
Unamortized loss on reacquired debt [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		$ 14.4	$ 14.8

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

[2] Prior year amount of $1.0 million reclassified from deferred other liabilities to Non-Current Regulatory Liabilities.

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