Stock Repurchases - Additional Information (Detail) - Class A Common Shares $ in Millions	9 Months Ended	173 Months Ended
	Jan. 31, 2017 USD ($) shares	Jan. 31, 2017 USD ($) shares
Equity, Class of Treasury Stock [Line Items]
Common stock shares repurchased	4,588,632
Cost of common stock repurchased | $	$ 25.6
Shares Stock Repurchase Plan, August 19, 2002
Equity, Class of Treasury Stock [Line Items]
Approved number of shares to be repurchased	2,000,000	2,000,000
Common stock shares repurchased		1,053,679
Cost of common stock repurchased | $		$ 6.2