Summary of Significant Accounting Policies, Regulated Operations (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012
Schedule of Regulatory Assets and Liabilities [Line Items]
Fuel clause under recoveries	$ 1.7		$ 0
Regulatory Assets, Current	29.8		27.0
Regulatory Assets, Non-Current	501.9		510.6
Fuel clause over recoveries	21.8		109.2
Regulatory Liability, Current	46.3		141.1
Regulatory Liability, Non-Current	244.4		245.1
Smart Grid rider over recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	19.4	[1]	24.1	[1]
Other [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	5.1	[1]	7.8	[1]
Accrued removal obligations, net [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Non-Current	219.7		218.2
Pension tracker [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Non-Current	12.6		9.2
Deferred pension credits [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Non-Current	12.1		17.7
Crossroads wind farm rider under recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	11.3	[2]	14.9	[2]
Oklahoma demand program rider under recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	8.4	[2]	9.2	[2]
Other [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	8.4	[2]	2.9	[2]
Regulatory Assets, Non-Current	14.3		12.3
Benefit obligations regulatory asset [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Non-Current	357.0		370.6
Income taxes recoverable from customers, net [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Non-Current	55.2		54.7
Smart Grid [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Non-Current	43.4		42.8
Deferred storm expenses [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Non-Current	17.2		12.7
Unamortized loss on reacquired debt [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Non-Current	12.4		13.0
Deferred pension expenses [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Non-Current	$ 2.4		$ 4.5

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

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