Summary of Significant Accounting Policies (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Abstract]
Depreciation expense expressed as a percentage of average depreciable property	2.90%	2.90%	2.80%
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 and Financing Receivable, after Allowance for Credit Loss, Current and Noncurrent [Abstract]
Accounts Receivable, Allowance for Credit Loss, Current	$ 5.3	$ 5.6
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	$ 31.0	33.9
Supplies
Public Utilities, Inventory [Line Items]
Inventory	24.7	25.7
Fuel
Public Utilities, Inventory [Line Items]
Inventory	$ 6.3	$ 8.2