Nonutility Legacy Holdings (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]
Restricted cash & other investments	$ 43.7	$ 34.1
Other nonutility investments	9.6	16.1
Nonutility Group
Schedule of Equity Method Investments [Line Items]
Debt security related to the sale of commercial real estate	5.1
Other nonutility investments	1.6
Legacy Investments, Other Than ProLiance | Nonutility Group
Schedule of Equity Method Investments [Line Items]
Restricted cash & other investments	$ 6.7	$ 7.0