Stockholders' Equity (Narrative) (Details) (USD $) In Millions, unless otherwise specified	0 Months Ended	1 Months Ended			3 Months Ended		6 Months Ended
	Apr. 26, 2012	Aug. 31, 2011	Apr. 30, 2011	Apr. 30, 2010	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Stockholders' Equity Note [Abstract]
Amount of common stock repurchases authorized		$ 400.0
Increase in stock repurchase authorized amount	150.0	250.0	150.0	150.0
Shares repurchased during period (shares)					2.2	1.5	2.4	2.6
Value of shares repurchased during period					67.2	50.5	75.1	93.3
Remaining amount available for future purchases of shares under approved repurchase program							104.9
Additional stock-based compensation					$ 1.9	$ 1.9	$ 3.7	$ 4.0