Selected Balance Sheet Data, Property, Plant and Equipment (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 3,718.6	$ 3,674.4
Less accumulated depreciation	(1,335.9)	(1,315.0)
Property, plant and equipment, net	2,382.7	2,359.4
Electric plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	3,048.0	3,024.6
Natural gas plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	370.4	365.5
Common and other property
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	205.4	201.5
Construction work in progress
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 94.8	$ 82.8