Fair Value of Financial Assets and Liabilities - Summary of Short-Term Investments (Details) - USD ($)	3 Months Ended
	Jul. 31, 2016	Jul. 31, 2015	Apr. 30, 2016
Schedule of Available-for-Sale Securities [Line Items]
Cost	$ 48,309,000		$ 50,692,000
Gross Unrealized Gains	12,000		5,000
Gross Unrealized Losses	(23,000)		(15,000)
Fair Value	48,298,000		50,682,000
Impairment Charges	0	$ 0
Certificates of deposit
Schedule of Available-for-Sale Securities [Line Items]
Cost	4,640,000		7,090,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	0		(1,000)
Fair Value	4,640,000		7,089,000
Commercial paper
Schedule of Available-for-Sale Securities [Line Items]
Cost	2,248,000		4,043,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	0		0
Fair Value	2,248,000		4,043,000
U.S. Treasury securities
Schedule of Available-for-Sale Securities [Line Items]
Cost	8,727,000		8,764,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	(15,000)		0
Fair Value	8,712,000		8,764,000
US government agencies debt securities
Schedule of Available-for-Sale Securities [Line Items]
Cost	25,508,000		24,841,000
Gross Unrealized Gains	11,000		4,000
Gross Unrealized Losses	(1,000)		(9,000)
Fair Value	25,518,000		24,836,000
Corporate debt securities
Schedule of Available-for-Sale Securities [Line Items]
Cost	7,186,000		5,954,000
Gross Unrealized Gains	1,000		1,000
Gross Unrealized Losses	(7,000)		(5,000)
Fair Value	$ 7,180,000		$ 5,950,000