Selected Balance Sheet Data, Property, Plant and Equipment (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 3,601.9	$ 3,485.9
Less accumulated depreciation	(1,297.6)	(1,244.3)
Property, plant and equipment, net	2,304.3	2,241.6
Electric plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	2,963.7	2,895.5
Natural gas plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	358.9	345.7
Common and other property
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	199.2	189.7
Construction work in progress
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 80.1	$ 55.0