Summary of Significant Accounting Policies Accounting Records (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Regulatory Assets, Noncurrent	$ 402.2	$ 410.4
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	(7.4)
Deferred Storm and Property Reserve Deficiency, Current	2.7
Smart Grid Project [Member]
Smart Grid Costs to be Recovered	6.9
Benefit Obligations [Member]
Regulatory Assets, Noncurrent	242.2	261.1
Regulatory Asset [Member]
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	21.2
Pension Plans, Defined Benefit [Member] | Defined Benefit Plans Income Loss [Member]
Components of Benefit Obligation Regulatory Asset	214.1	196.7
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	13.1
Pension Plans, Defined Benefit [Member] | Prior Service Cost [Member]
Components of Benefit Obligation Regulatory Asset	0.0	0.6
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	0.0
Other Postretirement Benefit Plans, Defined Benefit [Member] | Defined Benefit Plans Income Loss [Member]
Components of Benefit Obligation Regulatory Asset	34.2	83.6
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	2.0
Other Postretirement Benefit Plans, Defined Benefit [Member] | Prior Service Cost [Member]
Components of Benefit Obligation Regulatory Asset	(6.1)	$ (19.8)
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	$ 6.1