Earnings Per Common share - Diluted Earnings per Share for Class B Common Shares Using Two-Class Method (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands		3 Months Ended		9 Months Ended
		Jan. 31, 2019	Jan. 31, 2018	Jan. 31, 2019	Jan. 31, 2018
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Basic shares used in calculation of earnings per common share (in shares)		31,010	30,244	30,887	29,940
Diluted EPS for class B common shares (in shares)		31,183	30,701	31,351	30,299
Basic earnings per common share (in usd per share)	[1]	$ 0.07	$ 0.18	$ 0.16	$ 0.36
Diluted earnings per common share (in usd per share)	[1]	$ 0.07	$ 0.18	$ 0.16	$ 0.36
Class B Common Shares
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Undistributed & distributed earnings		$ 135	$ 409	$ 294	$ 806
Reallocation of undistributed earnings from Class A Common Shares to Class B Common Shares		1	(2)	1	(1)
Net earnings		$ 136	$ 407	$ 295	$ 805
Basic shares used in calculation of earnings per common share (in shares)		1,822	2,252	1,851	2,310
Diluted EPS for class B common shares (in shares)		1,822	2,252	1,851	2,310
Basic earnings per common share (in usd per share)		$ 0.07	$ 0.18	$ 0.16	$ 0.36
Diluted earnings per common share (in usd per share)		$ 0.07	$ 0.18	$ 0.16	$ 0.36

[1] (a)Basic per share amounts are the same for Class A and Class B shares. Diluted per share amounts for Class A shares are shown above. Diluted earnings per share for Class B shares under the two-class method are $0.07 and $0.18 for the three months ended January 31, 2019 and 2018, and $0.16 and $0.36 for the nine months ended January 31, 2019 and 2018, respectively. See Note D to the Condensed Consolidated Financial Statements.