Earnings Per Common share - Basic Earnings Per Common Share (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, Basic, by Common Class, Including Two Class Method [Line Items]
Total (in usd per share)	[1]	$ 0.07	$ 0.18	$ 0.16	$ 0.36
Basic weighted average common shares outstanding (in shares)		31,010	30,244	30,887	29,940
Class A Common Shares
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Distributed earnings (in usd per share)		$ 0.11	$ 0.11	$ 0.33	$ 0.33
Undistributed losses (in usd per share)		(0.04)	0.07	(0.17)	0.03
Total (in usd per share)		$ 0.07	$ 0.18	$ 0.16	$ 0.36
Distributed earnings (in usd)		$ 3,213	$ 3,102	$ 9,609	$ 9,180
Undistributed losses (in usd)		(1,047)	2,069	(4,974)	799
Total (in usd)		$ 2,166	$ 5,171	$ 4,635	$ 9,979
Basic weighted average common shares outstanding (in shares)		29,188	27,992	29,036	27,630
Class B Common Shares
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Distributed earnings (in usd per share)		$ 0.11	$ 0.11	$ 0.33	$ 0.33
Undistributed losses (in usd per share)		(0.04)	0.07	(0.17)	0.03
Total (in usd per share)		$ 0.07	$ 0.18	$ 0.16	$ 0.36
Distributed earnings (in usd)		$ 204	$ 242	$ 613	$ 747
Undistributed losses (in usd)		(69)	167	(319)	59
Total (in usd)		$ 135	$ 409	$ 294	$ 806
Basic weighted average common shares outstanding (in shares)		1,822	2,252	1,851	2,310

[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.