Earnings Per Common share - Diluted Earnings per Share for Class A Common Shares Using If-Converted 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, Class A 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 A Common Shares
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Undistributed & distributed earnings		$ 2,166	$ 5,171	$ 4,635	$ 9,979
Undistributed & distributed earnings, class B conversion (in usd per share)		135	409	294	806
Net earnings		$ 2,301	$ 5,580	$ 4,929	$ 10,785
Basic shares used in calculation of earnings per common share (in shares)		29,188	27,992	29,036	27,630
Common Stock Equivalents (in shares)		173	457	464	359
Weighted Average Number of Shares Including Common Stock Equivalents, Diluted (in shares)		29,361	28,449	29,500	27,989
Class B Conversion (in shares)		1,822	2,252	1,851	2,310
Diluted, Class A Common Shares (in shares)		31,183	30,701	31,351	30,299
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.