Fair Value of Financial Assets and Liabilities - Summary of Short-Term Investments (Details) - USD ($)	6 Months Ended
	Oct. 31, 2017	Oct. 31, 2016	Apr. 30, 2017
Schedule of Available-for-Sale Securities [Line Items]
Cost	$ 13,233,000		$ 38,718,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	(9,000)		(29,000)
Fair Value	13,224,000		38,689,000
Impairment Charges	0	$ 0
Certificates of deposit
Schedule of Available-for-Sale Securities [Line Items]
Cost	2,410,000		1,930,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	0		0
Fair Value	2,410,000		1,930,000
Commercial paper
Schedule of Available-for-Sale Securities [Line Items]
Cost	2,268,000		4,206,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	0		0
Fair Value	2,268,000		4,206,000
U.S. Treasury securities
Schedule of Available-for-Sale Securities [Line Items]
Cost			8,705,000
Gross Unrealized Gains			0
Gross Unrealized Losses			(5,000)
Fair Value			8,700,000
US government agencies debt securities
Schedule of Available-for-Sale Securities [Line Items]
Cost	5,248,000		16,733,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	(6,000)		(15,000)
Fair Value	5,242,000		16,718,000
Corporate debt securities
Schedule of Available-for-Sale Securities [Line Items]
Cost	3,307,000		7,144,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	(3,000)		(9,000)
Fair Value	$ 3,304,000		$ 7,135,000