Consolidated Balance Sheets (unaudited) (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Trade accounts receivable, less allowance of $11.7 (2017—$11.7)	$ 11.7	$ 11.7
Properties and equipment, net of accumulated depreciation and amortization of $1,497.9 (2017—$1,463.4)	$ 1,497.9	$ 1,463.4
Common stock issued, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock issued, shares authorized	300,000,000	300,000,000
Common stock issued, shares outstanding	67,295,704	67,780,410
Treasury stock, at cost (shares)	10,160,923	9,676,217