Presentation and Summary of Significant Accounting Policies - Basic Earnings per Common Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands		12 Months Ended
		Apr. 30, 2018	Apr. 30, 2017	Apr. 30, 2016
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Total per share	[1]	$ 0.40	$ 0.50	$ 0.36
Basic weighted average common shares		30,080	29,232	28,727
Class A Common Shares
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Distributed earnings per share		$ 0.44	$ 0.44	$ 0.40
Undistributed earnings/(loss) per share		(0.04)	0.06	(0.04)
Total per share	[2]	$ 0.40	$ 0.50	$ 0.36
Distributed earnings		$ 12,321	$ 11,845	$ 10,479
Undistributed earnings/(loss)		(1,151)	1,568	(1,148)
Total		$ 11,170	$ 13,413	$ 9,331
Basic weighted average common shares		27,805	26,793	26,143
Class B Common Shares
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Distributed earnings per share		$ 0.43	$ 0.44	$ 0.40
Undistributed earnings/(loss) per share		(0.04)	0.06	(0.04)
Total per share	[2]	$ 0.39	$ 0.50	$ 0.35
Distributed earnings		$ 974	$ 1,066	$ 1,025
Undistributed earnings/(loss)		(91)	142	(114)
Total		$ 883	$ 1,208	$ 911
Basic weighted average common shares		2,275	2,439	2,584

[1] 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.

[2] Amounts adjusted for rounding