Property Plant and Equipment Property Plant and Equipment (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property, plant and equipment	$ 8,075.0	$ 7,117.2
Property, plant and equipment, net	5,946.4	5,095.6
Electric plant
Property, Plant and Equipment [Line Items]
Property, plant and equipment	7,227.7	6,765.3
CWIP
Property, Plant and Equipment [Line Items]
Property, plant and equipment	847.3	351.9
Nuclear Fuel [Member]
Property, Plant and Equipment [Line Items]
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	$ 2,128.6	$ 2,021.6