Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Statement of Financial Position [Abstract]
Accounts receivable and unbilled revenues, reserves	$ 220.1	$ 140.0
Property, plant, and equipment, accumulated depreciation and amortization	$ 9,364.7	$ 8,878.7
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	325,000,000	325,000,000
Common stock, shares outstanding	315,434,531	315,434,531