Subsequent Event (Details) (USD $) In Millions	1 Months Ended
	Aug. 31, 2011	Apr. 30, 2011	Apr. 30, 2010	Apr. 30, 2009
Subsequent Event [Line Items]
Increase in stock repurchase authorized amount	$ 250.0	$ 150.0	$ 150.0
Amount of common stock repurchases authorized	$ 400.0			$ 100.0