Fair Value of Financial Assets and Liabilities - Summary of Short-Term Investments (Details) - USD ($)	9 Months Ended
	Jan. 31, 2017	Jan. 31, 2016	Apr. 30, 2016
Schedule of Available-for-Sale Securities [Line Items]
Cost	$ 45,233,000		$ 50,692,000
Gross Unrealized Gains	1,000		5,000
Gross Unrealized Losses	(27,000)		(15,000)
Fair Value	45,207,000		50,682,000
Impairment Charges	0	$ 0
Certificates of deposit
Schedule of Available-for-Sale Securities [Line Items]
Cost	3,670,000		7,090,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	0		(1,000)
Fair Value	3,670,000		7,089,000
Municipal debt securities
Schedule of Available-for-Sale Securities [Line Items]
Cost	541,000
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	541,000
Commercial paper
Schedule of Available-for-Sale Securities [Line Items]
Cost	4,446,000		4,043,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	0		0
Fair Value	4,446,000		4,043,000
U.S. Treasury securities
Schedule of Available-for-Sale Securities [Line Items]
Cost	8,712,000		8,764,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	(7,000)		0
Fair Value	8,705,000		8,764,000
US government agencies debt securities
Schedule of Available-for-Sale Securities [Line Items]
Cost	19,731,000		24,841,000
Gross Unrealized Gains	1,000		4,000
Gross Unrealized Losses	(7,000)		(9,000)
Fair Value	19,725,000		24,836,000
Corporate debt securities
Schedule of Available-for-Sale Securities [Line Items]
Cost	8,133,000		5,954,000
Gross Unrealized Gains	0		1,000
Gross Unrealized Losses	(13,000)		(5,000)
Fair Value	$ 8,120,000		$ 5,950,000