Consolidated Balance Sheets (unaudited) (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Statement of Financial Position [Abstract]
Trade accounts receivable, less allowance	$ 2.4	$ 1.4
Properties and equipment, net of accumulated depreciation and amortization	$ 1,337.1	$ 1,287.4
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	70,253,231	70,533,515
Treasury stock, at cost (shares)	7,203,394	6,923,110