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Summary of Significant Accounting Policies Accounting Records (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Components of Benefit Obligation Regulatory Asset	$ 261.1	$ 227.4
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	(4.1)
Deferred Storm and Property Reserve Deficiency, Current	2.7
Smart Grid Project [Member]
Smart Grid Costs to be Recovered	6.9
Regulatory Asset [Member]
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	11.3
Pension Plans, Defined Benefit [Member] \| Defined Benefit Plans Income Loss [Member]
Components of Benefit Obligation Regulatory Asset	196.7	178.4
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	12.7
Pension Plans, Defined Benefit [Member] \| Prior Service Cost [Member]
Components of Benefit Obligation Regulatory Asset	0.6	2.5
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	0.5
Other Postretirement Benefit Plans, Defined Benefit [Member] \| Defined Benefit Plans Income Loss [Member]
Components of Benefit Obligation Regulatory Asset	83.6	79.9
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	11.8
Other Postretirement Benefit Plans, Defined Benefit [Member] \| Prior Service Cost [Member]
Components of Benefit Obligation Regulatory Asset	(19.8)	(33.4)
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	$ (13.7)