Condensed Consolidated Balance Sheets (Parenthetical) (USD $) In Millions, except Per Share data, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	100.0	100.0
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	500.0	500.0
Common stock, shares issued	189.9	189.5
Treasury stock, shares	25.4	23.8