Components of Investments (Detail) - USD ($) $ in Thousands	Apr. 30, 2017	Apr. 30, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total debt securities	$ 14,803	$ 21,683
Marketable equity securities	8,984	7,198
Trading Securities, Total	23,787	28,881
Tax-exempt state and municipal bonds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total debt securities	14,803	21,487
Corporate Bonds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total debt securities	$ 0	$ 196