Stock Repurchases - Additional Information (Detail) (Class A Common Shares, USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended	143 Months Ended
	Jul. 31, 2014	Jul. 31, 2014 Shares Stock Repurchase Plan, August 19, 2002
Equity, Class of Treasury Stock [Line Items]
Approved number of shares to be repurchased		2,000,000
Common stock shares repurchased	4,444,815	909,862
Cost of common stock repurchased	$ 24.3	$ 4.9