Summary of Significant Accounting Policies, Regulated Operations (Details) - USD ($) $ in Millions		Jun. 30, 2015	Dec. 31, 2014
Schedule of Regulatory Assets and Liabilities [Line Items]
Fuel clause under recoveries		$ 3.7	$ 68.3
Fuel clause over recoveries		1.6	0.0
Total Current Regulatory Assets		42.7	97.1
Total Non-Current Regulatory Assets		404.8	411.5
Total Current Regulatory Liabilities		21.4	24.4
Total Non-Current Regulatory Liabilities		279.6	263.0
Crossroads Wind Farm Rider Over Recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Liabilities	[1]	8.6	10.3
Smart Grid rider over recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Liabilities	[1]	8.0	12.5
Other [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Liabilities	[1]	3.2	1.6
Accrued removal obligations, net
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities		256.4	248.1
Pension tracker
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities		23.2	14.9
Oklahoma demand program rider under recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Assets	[2]	26.6	19.7
Other [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Assets	[2]	12.4	9.1
Total Non-Current Regulatory Assets		18.0	16.8
Benefit obligations regulatory asset
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets		254.2	261.1
Income taxes recoverable from customers, net
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets		56.3	56.1
Smart Grid
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets		43.8	43.9
Deferred storm expenses
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets		17.0	17.5
Unamortized loss on reacquired debt
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets		$ 15.5	$ 16.1

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

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