CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Statement of Financial Position [Abstract]
Accounts receivable and unbilled revenues, reserves	$ 259.1	$ 220.1
Property, plant, and equipment, accumulated depreciation and amortization	$ 9,481.4	$ 9,364.7
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	315,434,531	315,434,531