CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Accounts receivable and accrued unbilled revenues, reserves	$ 163.2	$ 149.2
Property, plant, and equipment, accumulated depreciation	$ 8,589.0	$ 8,636.6
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized	325,000,000	325,000,000
Common stock, shares outstanding	315,438,398	315,523,192