Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Accounts receivable and unbilled revenues, reserves	$ 149.2	$ 143.2
Property, plant, and equipment, accumulated depreciation	$ 8,515.9	$ 8,618.5
Common stock, par value	$ 0.01
Common stock, shares authorized	325,000,000
Common stock, shares outstanding	315,523,192	315,574,624