Consolidated Balance Sheets (Parenthetical) (USD $) In Millions, except Per Share data	Jun. 30, 2011	Dec. 31, 2010
Current assets:
Allowance for doubtful accounts	$ 47.1	$ 50.9
Stockholders' equity:
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	50	50
Preferred stock, shares issued	0	0
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	750	750
Common stock, shares issued	401.6	400.2
Treasury stock, shares	25.1	16.5