Selected Balance Sheet Data, Property, Plant and Equipment (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 3,829.1	$ 3,674.4
Less accumulated depreciation	(1,376.7)	(1,315.0)
Property, plant and equipment, net	2,452.4	2,359.4
Electric plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	3,104.3	3,024.6
Natural gas plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	383.1	365.5
Common and other property
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	204.9	201.5
Construction work in progress
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 136.8	$ 82.8