Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Accounts receivable and unbilled revenues, reserves	$ 140.0	$ 149.2
Property, plant, and equipment, accumulated depreciation	$ 8,878.7	$ 8,515.9
Common stock, par value	$ 0.01
Common stock, shares authorized	325,000,000
Common stock, shares outstanding	315,434,531	315,523,192