Fair Value of Financial Assets and Liabilities - Summary of Short-Term Investments (Details) - USD ($)	12 Months Ended
	Apr. 30, 2016	Apr. 30, 2015
Schedule of Available-for-Sale Securities [Line Items]
Cost	$ 50,692,000	$ 52,758,000
Gross Unrealized Gains	5,000	30,000
Gross Unrealized Losses	(15,000)	(58,000)
Fair Value	50,682,000	52,730,000
Impairment Charges	0	0
Certificates of deposit
Schedule of Available-for-Sale Securities [Line Items]
Cost	7,090,000	4,000,000
Gross Unrealized Gains	0	4,000
Gross Unrealized Losses	(1,000)	0
Fair Value	7,089,000	4,004,000
Municipal debt securities
Schedule of Available-for-Sale Securities [Line Items]
Cost		4,564,000
Gross Unrealized Gains		17,000
Gross Unrealized Losses		(16,000)
Fair Value		4,565,000
Commercial paper
Schedule of Available-for-Sale Securities [Line Items]
Cost	4,043,000	6,269,000
Gross Unrealized Gains	0	4,000
Gross Unrealized Losses	0	0
Fair Value	4,043,000	6,273,000
U.S. Treasury securities
Schedule of Available-for-Sale Securities [Line Items]
Cost	8,764,000	11,814,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(11,000)
Fair Value	8,764,000	11,803,000
US government agencies debt securities
Schedule of Available-for-Sale Securities [Line Items]
Cost	24,841,000	17,007,000
Gross Unrealized Gains	4,000	1,000
Gross Unrealized Losses	(9,000)	(3,000)
Fair Value	24,836,000	17,005,000
Corporate debt securities
Schedule of Available-for-Sale Securities [Line Items]
Cost	5,954,000	9,104,000
Gross Unrealized Gains	1,000	4,000
Gross Unrealized Losses	(5,000)	(28,000)
Fair Value	$ 5,950,000	$ 9,080,000