Regulatory Matters (Regulatory Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 2,660.0	$ 2,795.6
Overrecovered Gas And Fuel Costs
Regulatory Liabilities [Line Items]
Regulatory Liabilities	32.0	27.6
Cost Of Removal
Regulatory Liabilities [Line Items]
Regulatory Liabilities	1,076.0	1,096.8
Regulatory Effects Of Accounting For Income Taxes
Regulatory Liabilities [Line Items]
Regulatory Liabilities	1,428.3	1,563.4
Other Postretirement Costs
Regulatory Liabilities [Line Items]
Regulatory Liabilities	62.7	59.0
Other Liabilities
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 61.0	$ 48.8