Summary of Significant Accounting Policies Accounting Records (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Regulatory Assets, Current	$ 18.5	$ 40.8
Regulatory Assets, Noncurrent	285.8	283.0
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	(2.3)
Deferred Storm and Property Reserve Deficiency, Current	2.7
Regulatory Liability, Current	36.6	3.9
Pension Plans, Defined Benefit [Member] | Defined Benefit Plans Income Loss [Member]
Components of Benefit Obligation Regulatory Asset	185.3	172.4
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	13.8
Other Postretirement Benefit Plans, Defined Benefit [Member] | Defined Benefit Plans Income Loss [Member]
Components of Benefit Obligation Regulatory Asset	25.6	33.6
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	2.7
Other Postretirement Benefit Plans, Defined Benefit [Member] | Prior Service Cost [Member]
Components of Benefit Obligation Regulatory Asset	(22.7)	$ (28.8)
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	(6.1)
Regulatory Asset [Member]
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	$ 10.4