Summary of Significant Accounting Policies, Regulated Operations (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013
Schedule of Regulatory Assets and Liabilities [Line Items]
Fuel clause under recoveries	$ 62.0		$ 26.2
Total Current Regulatory Assets	80.0		48.8
Total Non-Current Regulatory Assets	374.4		379.1
Fuel clause over recoveries	0		0.4
Total Current Regulatory Liabilities	18.3		20.2
Total Non-Current Regulatory Liabilities	237.6		234.2
Smart Grid rider over recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Liabilities	15.7	[1]	16.7	[1]
Other [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Liabilities	2.6	[1]	3.1	[1]
Accrued removal obligations, net
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities	230.8		227.7
Deferred pension credits
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities	3.7		6.5
Pension tracker
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities	3.1		0
Oklahoma demand program rider under recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Assets	8.8	[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	9.2	[2]	7.3	[2]
Total Non-Current Regulatory Assets	16.1		16.2
Benefit obligations regulatory asset
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	224.8		227.4
Income taxes recoverable from customers, net
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	56.4		56.5
Smart Grid
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	44.1		44.2
Deferred storm expenses
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	21.5		21.6
Unamortized loss on reacquired debt
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	11.5		11.8
Pension tracker
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.