Consolidated Balance Sheet (Parenthetical) (USD $) In Millions, except Share data, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Statement of Financial Position [Abstract]
Trade accounts receivable, allowance	$ 5.2	$ 8.1
Properties and equipment, accumulated depreciation and amortization	$ 1,785.1	$ 1,722.7
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	75,565,409	73,886,050
Treasury stock, shares	1,414,351	3,093,710