Summary of Significant Accounting Policies, Regulated Operations (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013
Schedule of Regulatory Assets and Liabilities [Line Items]
Fuel clause under recoveries	$ 84.3		$ 26.2
Total Current Regulatory Assets	110.8		48.8
Total Non-Current Regulatory Assets	372.6		379.1
Fuel clause over recoveries	0	[1]	0.4	[1]
Total Current Regulatory Liabilities	26.8		20.2
Total Non-Current Regulatory Liabilities	266.6		234.2
Smart Grid rider over recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Liabilities	14.5	[1]	16.7	[1]
Crossroads Wind Farm Rider Over Recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Liabilities	10.7	[1]	0	[1]
Other [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Liabilities	1.6	[1]	3.1	[1]
Accrued removal obligations, net
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities	255.7		227.7
Pension tracker
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities	10.9		0
Deferred pension credits
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities	0		6.5
Oklahoma demand program rider under recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Assets	17.8	[2]	10.6	[2]
Crossroads wind farm rider under recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Assets	0	[2]	4.7	[2]
Other [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Assets	8.7	[2]	7.3	[2]
Total Non-Current Regulatory Assets	17.2		16.2
Benefit obligations regulatory asset
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	219.4		227.4
Income taxes recoverable from customers, net
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	56.1		56.5
Smart Grid
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	44.0		44.2
Deferred storm expenses
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	19.4		21.6
Unamortized loss on reacquired debt
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	16.5		11.8
Deferred pension credits
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	$ 0		$ 1.4

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

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