Consolidated Balance Sheet (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Statement of Financial Position [Abstract]
Trade accounts receivable, allowance (in dollars)	$ 7.7	$ 5.8
Properties and equipment, accumulated depreciation and amortization	$ 1,746.5	$ 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	70,533,515	72,922,565
Treasury Stock, Shares	6,923,110	4,524,688