Utility & Nonutility Plant - Nonutility Plant (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Nonutility plant - net	$ 464.1	$ 423.9
Nonutility Group
Property, Plant and Equipment [Line Items]
Nonutility plant - net	464.1	423.9
Accumulated depreciation and amortization	506.9	460.8
Capitalized interest on nonutility plant construction projects	1.2	1.0	$ 0.4
Nonutility Group | Change in Accounting Method Accounted for as Change in Estimate | Infrastructure Services
Property, Plant and Equipment [Line Items]
Depreciation		9.6
Nonutility Group | Vehicles & equipment
Property, Plant and Equipment [Line Items]
Nonutility plant - net	220.2	207.4
Nonutility Group | Computer hardware & software
Property, Plant and Equipment [Line Items]
Nonutility plant - net	156.5	121.8
Nonutility Group | Land & buildings
Property, Plant and Equipment [Line Items]
Nonutility plant - net	77.1	77.9
Nonutility Group | All other
Property, Plant and Equipment [Line Items]
Nonutility plant - net	$ 10.3	$ 16.8