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, Gross	$ 3,485.9	$ 3,259.3
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	1,244.3	1,170.6
Property, Plant and Equipment, Net	2,241.6	2,088.7
Electric plant
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	2,895.5	2,602.7
Natural gas plant
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	345.7	326.7
Common and other property
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	189.7	181.1
CWIP
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 55.0	$ 148.8