Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Statement of Financial Position [Abstract]
Accounts receivable and unbilled revenues, reserves	$ 108.0	$ 113.3
Property, plant, and equipment, accumulated depreciation	$ 8,214.6	$ 7,919.1
Common stock, par value	$ 0.01
Common stock, shares authorized	325,000,000
Common stock, shares outstanding	315,614,941	315,683,496