Summary of Significant Accounting Policies Regulated Operations (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014
Schedule of Regulatory Assets and Liabilities [Line Items]
Fuel clause under recoveries		$ 0.0	$ 68.3
Fuel clause over recoveries		61.3	0.0
Regulatory Assets, Current		46.5	98.2
Regulatory Assets, Noncurrent		402.2	410.4
Regulatory Liability, Current		68.8	24.4
Regulatory Liability, Noncurrent		272.6	263.0
Prior Period Reclassification Adjustment		1.1
Smart Grid rider over recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	[1]	2.0	12.5
Crossroads Wind Farm Rider Over Recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	[1]	2.9	10.3
Other Regulatory Liabilities [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	[1]	2.6	1.6
Accrued removal obligations [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Noncurrent		254.9	248.1
Pension tracker [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Noncurrent		17.7	14.9
Oklahoma demand program rider under recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	[2]	36.6	19.7
Other Regulatory Assets [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	[2],[3]	9.9	10.2
Regulatory Assets, Noncurrent	[3]	17.3	15.7
Benefit obligations regulatory asset [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		242.2	261.1
Income taxes recoverable from customers, net [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		56.7	56.1
Smart Grid [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		43.6	43.9
Deferred storm expenses [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		27.6	17.5
Unamortized loss on reacquired debt [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent		$ 14.8	$ 16.1

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

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

[3] Prior year amount of $1.1 million reclassified from Non-Current Other assets to Current Other assets.