Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Jul. 31, 2019	Apr. 30, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 59,776	$ 56,645
Marketable securities	25,270	27,194
Total	85,046	83,839
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	59,776	56,645
Marketable securities	10,902	11,002
Total	70,678	67,647
Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Marketable securities	14,368	16,192
Total	14,368	16,192
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Marketable securities	0	0
Total	$ 0	$ 0