Summary of Significant Accounting Policies (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Abstract]
Depreciation expense expressed as a percentage of average depreciable property	2.90%	2.80%	2.70%
Cash and Cash Equivalents [Abstract]
Maximum number of months of remaining maturity at time of purchase to consider investments in certain instruments as cash equivalents	three months
Accounts, Notes, Loans and Financing Receivable, Classified [Abstract]
Allowance for Doubtful Accounts Receivable, Current	$ 5.6	$ 6.3
Alternative Revenue Programs [Abstract]
Maximum number of months following end of annual period in which revenues are earned to be included in incentive programs	24 months
Public Utilities, Inventory [Line Items]
Inventory	$ 33.9	40.4
Supplies
Public Utilities, Inventory [Line Items]
Inventory	25.7	26.2
Fuel
Public Utilities, Inventory [Line Items]
Inventory	$ 8.2	$ 14.2