Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Oct. 31, 2019	Apr. 30, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 57,698	$ 56,645
Marketable securities	31,987	27,194
Total	89,685	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	57,698	56,645
Marketable securities	12,478	11,002
Total	70,176	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	19,509	16,192
Total	19,509	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