Summary of Significant Accounting Policies Accounting Records (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Components of Benefit Obligation Regulatory Asset	$ 232.6	$ 242.2
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	14.7
Pension Plans [Member] | Defined Benefit Plans Income Loss [Member]
Components of Benefit Obligation Regulatory Asset	199.9	214.1
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	12.4
Other Postretirement Benefit Plans, Defined Benefit [Member] | Defined Benefit Plans Income Loss [Member]
Components of Benefit Obligation Regulatory Asset	32.7	34.2
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	2.3
Other Postretirement Benefit Plans, Defined Benefit [Member] | Prior Service Cost [Member]
Components of Benefit Obligation Regulatory Asset	$ 0.0	$ (6.1)