Financial Statements and Supplementary Data - Schedule of Quarterly Results (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended																12 Months Ended
	Apr. 30, 2018		Jan. 31, 2018		Oct. 31, 2017		Jul. 31, 2017		Apr. 30, 2017		Jan. 31, 2017		Oct. 31, 2016		Jul. 31, 2016		Apr. 30, 2018		Apr. 30, 2017		Apr. 30, 2016
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 29,363		$ 30,117		$ 26,337		$ 26,886		$ 26,266		$ 26,441		$ 26,146		$ 27,433		$ 112,703		$ 106,286		$ 113,889
Gross margin	16,426		17,046		14,006		15,225		14,340		14,049		13,018		13,796		62,703		55,202		59,660
Operating income	2,463		4,204		3,242		3,622		2,957		2,455		708		1,646		13,531		7,766		13,527
Net earnings	$ 1,268		$ 5,580		$ 2,480		$ 2,725		$ 10,278		$ 2,243		$ 412		$ 1,688		$ 12,053		$ 14,621		$ 10,242
Diluted earnings per share	$ 0.04	[1]	$ 0.18	[1]	$ 0.08	[1]	$ 0.09	[1]	$ 0.34	[1]	$ 0.08	[1]	$ 0.01	[1]	$ 0.06	[1]	$ 0.40	[1],[2]	$ 0.49	[1],[2]	$ 0.35	[2]

[1] Table may contain rounding

[2] Diluted per share amounts for Class A shares are shown above. Diluted per share for Class B shares under the two-class method are $0.39, $0.49 and $0.35 for the years ended April 30, 2018, 2017 and 2016, respectively. See Note 1 to the Consolidated Financial Statements.