CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Accounts receivable and accrued unbilled revenues, reserves	$ 112.7	$ 108.0
Property, plant, and equipment, accumulated depreciation	$ 8,525.5	$ 8,214.6
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	315,575,562	315,614,941