Property, Plant and Equipment (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 22,441.6	$ 21,815.4
Accumulated depreciation and amortization	(8,181.0)	(7,945.3)
Property, plant and equipment, net	14,584.8	14,244.0
Electric plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	19,054.8	18,519.5
Natural gas plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	1,593.2	1,562.9
Common and other property
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	908.2	886.7
Construction work in progress
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	885.4	846.3
Nuclear fuel
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	2,924.8	2,909.8
Accumulated depreciation and amortization	$ (2,600.6)	$ (2,535.9)