Fair Value of Financial Assets and Liabilities - Summary of Short-Term Investments (Details) - USD ($)	6 Months Ended
	Oct. 31, 2015	Oct. 31, 2014	Apr. 30, 2015
Schedule of Available-for-Sale Securities [Line Items]
Cost	$ 51,966,000		$ 52,758,000
Gross Unrealized Gains	1,000		30,000
Gross Unrealized Losses	(25,000)		(58,000)
Fair Value	51,942,000		52,730,000
Impairment Charges	0	$ 0
Certificates of deposit
Schedule of Available-for-Sale Securities [Line Items]
Cost	9,060,000		4,000,000
Gross Unrealized Gains	0		4,000
Gross Unrealized Losses	(2,000)		0
Fair Value	9,058,000		4,004,000
Municipal debt securities
Schedule of Available-for-Sale Securities [Line Items]
Cost	395,000		4,564,000
Gross Unrealized Gains	0		17,000
Gross Unrealized Losses	0		(16,000)
Fair Value	395,000		4,565,000
Commercial paper
Schedule of Available-for-Sale Securities [Line Items]
Cost	3,588,000		6,269,000
Gross Unrealized Gains	0		4,000
Gross Unrealized Losses	0		0
Fair Value	3,588,000		6,273,000
U.S. Treasury securities
Schedule of Available-for-Sale Securities [Line Items]
Cost	8,774,000		11,814,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	(5,000)		(11,000)
Fair Value	8,769,000		11,803,000
US government agencies debt securities
Schedule of Available-for-Sale Securities [Line Items]
Cost	21,192,000		17,007,000
Gross Unrealized Gains	1,000		1,000
Gross Unrealized Losses	(8,000)		(3,000)
Fair Value	21,185,000		17,005,000
Corporate debt securities
Schedule of Available-for-Sale Securities [Line Items]
Cost	8,957,000		9,104,000
Gross Unrealized Gains	0		4,000
Gross Unrealized Losses	(10,000)		(28,000)
Fair Value	$ 8,947,000		$ 9,080,000