Summary of Significant Accounting Policies Accounting Records (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Components of Benefit Obligation Regulatory Asset	$ 359.2	$ 365.5
Smart Grid Pilot Cost Cap	366.4
Incremental Costs Cap	6.9
Deferred Storm and Property Reserve Deficiency, Current	2.7
Regulatory Assets, Non - Current	507.9	489.4
Operations and Maintenance Expenses [Member]
Deferred Storm and Property Reserve Deficiency, Current	2.7
Defined Benefit Plan Prior Service Cost [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Components of Benefit Obligation Regulatory Asset	(60.8)	0
Defined Benefit Plan Prior Service Cost [Member] | Pension Plans, Defined Benefit [Member]
Components of Benefit Obligation Regulatory Asset	7.0	9.7
Defined Benefit Plan Net Transition Obligation [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Components of Benefit Obligation Regulatory Asset	2.5	5.1
Defined Benefit Plans Income Loss [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Components of Benefit Obligation Regulatory Asset	144.2	135.7
Defined Benefit Plans Income Loss [Member] | Pension Plans, Defined Benefit [Member]
Components of Benefit Obligation Regulatory Asset	266.3	215.0
Defined Benefit Plan Prior Service Cost [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	(1.8)
Defined Benefit Plan Prior Service Cost [Member] | Pension Plans, Defined Benefit [Member]
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	0.3
Defined Benefit Plan Net Transition Obligation [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	0.1
Defined Benefit Plans Income Loss [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	1.9
Defined Benefit Plans Income Loss [Member] | Pension Plans, Defined Benefit [Member]
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	2.7
Incremental Costs [Member]
Smart Grid Costs to be Recovered	0.8
Regulatory Asset [Domain]
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	27.4
Regulatory Asset [Domain] | Defined Benefit Plan Prior Service Cost [Member] | Pension Plans, Defined Benefit [Member]
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	2.5
Regulatory Asset [Domain] | Defined Benefit Plans Income Loss [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	17.3
Regulatory Asset [Domain] | Defined Benefit Plans Income Loss [Member] | Pension Plans, Defined Benefit [Member]
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	18.8
Regulatory Asset [Domain] | Defined Benefit Plan Prior Service Cost [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	(13.7)
Regulatory Asset [Domain] | Defined Benefit Plan Net Transition Obligation [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	2.5
Operations and Maintenance Expenses [Member]
Smart Grid Costs to be Recovered	14.0
OKLAHOMA
Regulatory Assets, Non - Current	16.8
ARKANSAS
Regulatory Assets, Non - Current	$ 3.2