Consolidated Balance Sheets (unaudited) (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Statement of Financial Position [Abstract]
Trade accounts receivable, less allowance	$ 1.8	$ 1.4
Properties and equipment, net of accumulated depreciation and amortization	$ 1,306.3	$ 1,316.4
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	70,533,749	70,533,515
Treasury stock, at cost (shares)	6,922,876	6,923,110