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	3.70%	3.60%	3.60%
Nuclear Decommissioning [Abstract]
Minimum amount of time between nuclear decommissioning studies (in years)	3 years
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	3 months
Accounts and Financing Receivable, after Allowance for Credit Loss, Current and Noncurrent [Abstract]
Allowance for bad debts	$ 23.0	$ 23.5
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]
Inventories	$ 304.8	299.4
Supplies
Public Utilities, Inventory [Line Items]
Inventories	175.6	176.3
Public Utilities, Inventory, Fuel
Public Utilities, Inventory [Line Items]
Inventories	103.2	88.5
Public Utilities, Inventory, Natural Gas
Public Utilities, Inventory [Line Items]
Inventories	$ 26.0	$ 34.6