Consolidated Balance Sheets (Parenthetical) (USD $) In Millions, except Share data, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Statement of Financial Position [Abstract]
Trade accounts receivable, allowance	$ 5.2	$ 5.2
Properties and equipment, accumulated depreciation and amortization (in dollars)	$ 1,807.2	$ 1,785.1
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	76,577,841	75,565,409
Treasury stock, shares	412,359	1,414,351