Consolidated Balance Sheets (unaudited) (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Allowance for doubtful accounts	$ 11.4	$ 11.7
Accumulated depreciation	$ 1,496.4	$ 1,463.4
Common stock issued, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock issued, shares authorized (in shares)	300,000,000	300,000,000
Common stock issued, shares outstanding (in shares)	67,098,411	67,780,410
Treasury stock, at cost (shares)	10,358,216	9,676,217