Property Plant and Equipment Property Plant and Equipment (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	$ 3,674.4	$ 3,485.9
Accumulated Depreciation	1,315.0	1,244.3
Property, Plant and Equipment, Net	2,359.4	2,241.6
Electric plant
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	3,024.6	2,895.5
Natural gas plant
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	365.5	345.7
Common and other property
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	201.5	189.7
CWIP
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	$ 82.8	$ 55.0