Stock Repurchase Program - Additional Information (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	1 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended
	May 25, 2011	Apr. 30, 2013	Apr. 30, 2012	Apr. 30, 2013	Apr. 30, 2012	Oct. 31, 2012 Accelerated Share Repurchase Agreement	Jan. 31, 2012 Accelerated Share Repurchase Agreement
Stock Repurchase Program [Line Items]
Stock repurchase program authorized amount	$ 500.0			$ 500.0
Accelerated share repurchase agreement amount							40.0
Average purchase price per share		$ 34.90	$ 32.04	$ 34.90	$ 29.64	$ 29.23
Remaining amount available for further repurchases				$ 237.4