Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Statement of Financial Position [Abstract]
Trade accounts receivable, less allowance	$ 7.0	$ 5.8
Properties and equipment, net of accumulated depreciation and amortization	$ 1,771.0	$ 1,818.4
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	72,418,302	72,922,565
Treasury stock, at cost: shares	5,038,323	4,524,688