Consolidated Balance Sheets (unaudited) (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Trade accounts receivable, less allowance of $12.6 (2016—$2.2)	$ 12.6	$ 2.2
Properties and equipment, net of accumulated depreciation and amortization of $1,438.2 (2016—$1,327.5)	$ 1,438.2	$ 1,327.5
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	67,758,485	68,309,431
Treasury stock, at cost (shares)	9,698,142	9,147,196