Summary of Significant Accounting Policies, Regulated Operations (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Schedule of Regulatory Assets and Liabilities [Line Items]
Fuel clause under recoveries	$ 0.4		$ 0
Regulatory Assets, Current	30.8		27.0
Regulatory Assets, Non-Current	502.6		510.6
Fuel clause over recoveries	81.6		109.2
Regulatory Liability, Current	109.7		141.1
Regulatory Liability, Non-Current	245.5		245.1
Smart Grid rider over recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	21.6	[1]	24.1	[1]
Other [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	6.5	[1]	7.8	[1]
Accrued removal obligations, net [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Non-Current	219.4		218.2
Deferred pension credits [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Non-Current	14.9		17.7
Pension tracker [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Non-Current	11.2		9.2
Crossroads wind farm rider under recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	13.4	[2]	14.9	[2]
Oklahoma demand program rider under recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	9.4	[2]	9.2	[2]
Other [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	7.6	[2]	2.9	[2]
Regulatory Assets, Non-Current	12.3		12.3
Benefit obligations regulatory asset [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Non-Current	364.1		370.6
Income taxes recoverable from customers, net [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Non-Current	54.9		54.7
Smart Grid [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Non-Current	42.9		42.8
Unamortized loss on reacquired debt [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Non-Current	12.7		13.0
Deferred storm expenses [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Non-Current	12.3		12.7
Deferred pension expenses [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Non-Current	$ 3.4		$ 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.