Summary of Significant Accounting Policies Accounting Records (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Components of Benefit Obligation Regulatory Asset	$ 370.6	$ 359.2
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	3.5
Smart Grid Project [Member]
Smart Grid Cost Cap	366.4
Operations and Maintenance Expenses [Member]
Deferred Storm and Property Reserve Deficiency, Current	2.7
Regulatory Asset [Member]
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	26.2
Pension Plans, Defined Benefit [Member] | Defined Benefit Plans Income Loss [Member]
Components of Benefit Obligation Regulatory Asset	278.6	266.3
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	19.8
Pension Plans, Defined Benefit [Member] | Prior Service Cost [Member]
Components of Benefit Obligation Regulatory Asset	4.5	7.0
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	2.0
Other Postretirement Benefit Plans, Defined Benefit [Member] | Defined Benefit Plans Income Loss [Member]
Components of Benefit Obligation Regulatory Asset	134.6	144.2
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	18.1
Other Postretirement Benefit Plans, Defined Benefit [Member] | Prior Service Cost [Member]
Components of Benefit Obligation Regulatory Asset	(47.1)	(60.8)
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	(13.7)
Other Postretirement Benefit Plans, Defined Benefit [Member] | Defined Benefit Plan Net Transition Obligation [Member]
Components of Benefit Obligation Regulatory Asset	0	2.5
OKLAHOMA | Web Portal Access and Education [Member] | Smart Grid Project [Member]
Smart Grid Costs to be Recovered	6.9
ARKANSAS | Smart Grid Project [Member]
Smart Grid Costs to be Recovered	14.0
ARKANSAS | Web Portal Access and Education [Member] | Smart Grid Project [Member]
Smart Grid Costs to be Recovered	$ 0.8