Fair Value of Financial Assets and Liabilities - Summary of Short-Term Investments (Details) - USD ($)	9 Months Ended
	Jan. 31, 2016	Jan. 31, 2015	Apr. 30, 2015
Schedule of Available-for-Sale Securities [Line Items]
Cost	$ 51,033,000		$ 52,758,000
Gross Unrealized Gains	0		30,000
Gross Unrealized Losses	(58,000)		(58,000)
Fair Value	50,975,000		52,730,000
Impairment Charges	0	$ 0
Certificates of deposit
Schedule of Available-for-Sale Securities [Line Items]
Cost	7,810,000		4,000,000
Gross Unrealized Gains	0		4,000
Gross Unrealized Losses	(1,000)		0
Fair Value	7,809,000		4,004,000
Municipal debt securities
Schedule of Available-for-Sale Securities [Line Items]
Cost	391,000		4,564,000
Gross Unrealized Gains	0		17,000
Gross Unrealized Losses	0		(16,000)
Fair Value	391,000		4,565,000
Commercial paper
Schedule of Available-for-Sale Securities [Line Items]
Cost	3,442,000		6,269,000
Gross Unrealized Gains	0		4,000
Gross Unrealized Losses	0		0
Fair Value	3,442,000		6,273,000
U.S. Treasury securities
Schedule of Available-for-Sale Securities [Line Items]
Cost	8,769,000		11,814,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	(8,000)		(11,000)
Fair Value	8,761,000		11,803,000
US government agencies debt securities
Schedule of Available-for-Sale Securities [Line Items]
Cost	25,078,000		17,007,000
Gross Unrealized Gains	0		1,000
Gross Unrealized Losses	(35,000)		(3,000)
Fair Value	25,043,000		17,005,000
Corporate debt securities
Schedule of Available-for-Sale Securities [Line Items]
Cost	5,543,000		9,104,000
Gross Unrealized Gains	0		4,000
Gross Unrealized Losses	(14,000)		(28,000)
Fair Value	$ 5,529,000		$ 9,080,000