Consolidated Balance Sheets (unaudited) (Parenthetical) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Trade accounts receivable, less allowance	$ 2.4	$ 2.2
Properties and equipment, net of accumulated depreciation and amortization	$ 1,397.6	$ 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	68,226,070	68,309,431
Treasury stock, at cost (shares)	9,230,557	9,147,196