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	$ 22,011.1	$ 21,815.4
Accumulated depreciation and amortization	(8,065.8)	(7,945.3)
Property, plant and equipment, net	14,290.1	14,244.0
Electric plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	18,548.7	18,519.5
Natural gas plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	1,578.0	1,562.9
Common and other property
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	903.9	886.7
Construction work in progress
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	980.5	846.3
Nuclear fuel
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	2,913.2	2,909.8
Accumulated depreciation and amortization	$ (2,568.4)	$ (2,535.9)