Summary of Significant Accounting Policies, Regulated Operations (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014		Dec. 31, 2013
Schedule of Regulatory Assets and Liabilities [Line Items]
Fuel clause under recoveries	$ 82.1		$ 26.2
Total Current Regulatory Assets	106.1		48.8
Total Non-Current Regulatory Assets	376.5		379.1
Fuel clause over recoveries	0		0.4
Total Current Regulatory Liabilities	20.0		20.2
Total Non-Current Regulatory Liabilities	239.7		234.2
Smart Grid rider over recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Liabilities	13.7	[1]	16.7	[1]
Other [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Liabilities	6.3	[1]	3.1	[1]
Accrued removal obligations, net
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities	232.0		227.7
Deferred pension credits
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities	0.9		6.5
Pension tracker
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities	6.8		0
Oklahoma demand program rider under recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Assets	12.8	[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	11.2	[2]	7.3	[2]
Total Non-Current Regulatory Assets	17.7		16.2
Benefit obligations regulatory asset
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	222.1		227.4
Income taxes recoverable from customers, net
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	56.2		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	20.7		21.6
Unamortized loss on reacquired debt
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets	15.7		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.