Summary of Significant Accounting Policies Regulated Operations (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Schedule of Regulatory Assets and Liabilities [Line Items]
Fuel clause under recoveries	$ 0		$ 1.8
Regulatory Assets, Current	27.0		16.0
Regulatory Assets, Noncurrent	510.6		507.9
Fuel clause over recoveries	109.2		7.7
Regulatory Liability, Current	141.1		45.7
Regulatory Liability, Noncurrent	245.1		230.7
Smart Grid rider over recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	24.1	[1]	24.3	[1]
Other Regulatory Liabilities [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	7.8	[1]	13.7	[1]
Accrued removal obligations [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Noncurrent	218.2		208.2
Deferred Pension Credits [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Noncurrent	17.7		0
Pension tracker [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Noncurrent	9.2		22.5
Crossroads wind farm rider under recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	14.9	[2]	2.5	[2]
Oklahoma demand program rider under recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	9.2	[2]	8.1	[2]
Other Regulatory Assets [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	2.9	[2]	3.6	[2]
Regulatory Assets, Noncurrent	12.3		10.4
Benefit Obligations [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	370.6		359.2
Income taxes recoverable from customers, net [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	54.7		54.0
Smart Grid [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	42.8		37.2
Unamortized loss on reacquired debt [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	13.0		14.2
Deferred storm expenses [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	12.7		23.8
Deferred Pension Expenses [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	$ 4.5		$ 9.1

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

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