Consolidated Balance Sheets (Parenthetical) (USD $) In Millions, except Share data, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Consolidated Balance Sheets
Trade accounts receivable, allowance (in dollars)	$ 8.2	$ 8.1
Properties and equipment, accumulated depreciation and amortization (in dollars)	$ 1,758.2	$ 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	74,569,101	73,886,050
Treasury stock, shares	2,410,659	3,093,710