Fair Value of Financial Assets and Liabilities - Summary of Short-Term Investments (Details) - USD ($)	6 Months Ended
	Oct. 31, 2016	Oct. 31, 2015	Apr. 30, 2016
Schedule of Available-for-Sale Securities [Line Items]
Cost	$ 41,715,000		$ 50,692,000
Gross Unrealized Gains	7,000		5,000
Gross Unrealized Losses	(18,000)		(15,000)
Fair Value	41,704,000		50,682,000
Impairment Charges	0	$ 0
Certificates of deposit
Schedule of Available-for-Sale Securities [Line Items]
Cost	2,470,000		7,090,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	0		(1,000)
Fair Value	2,470,000		7,089,000
Municipal debt securities
Schedule of Available-for-Sale Securities [Line Items]
Cost	546,000
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	546,000
Commercial paper
Schedule of Available-for-Sale Securities [Line Items]
Cost	848,000		4,043,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	0		0
Fair Value	848,000		4,043,000
U.S. Treasury securities
Schedule of Available-for-Sale Securities [Line Items]
Cost	8,720,000		8,764,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	(11,000)		0
Fair Value	8,709,000		8,764,000
US government agencies debt securities
Schedule of Available-for-Sale Securities [Line Items]
Cost	21,537,000		24,841,000
Gross Unrealized Gains	7,000		4,000
Gross Unrealized Losses	0		(9,000)
Fair Value	21,544,000		24,836,000
Corporate debt securities
Schedule of Available-for-Sale Securities [Line Items]
Cost	7,594,000		5,954,000
Gross Unrealized Gains	0		1,000
Gross Unrealized Losses	(7,000)		(5,000)
Fair Value	$ 7,587,000		$ 5,950,000