Fair Value of Financial Assets and Liabilities - Summary of Short-Term Investments (Details) - USD ($)	3 Months Ended
	Jul. 31, 2015	Jul. 31, 2014	Apr. 30, 2015
Schedule of Available-for-Sale Securities [Line Items]
Cost	$ 49,697,000		$ 52,758,000
Gross Unrealized Gains	43,000		30,000
Gross Unrealized Losses	(44,000)		(58,000)
Fair Value	49,696,000		52,730,000
Impairment Charges	0	$ 0
Certificates of deposit
Schedule of Available-for-Sale Securities [Line Items]
Cost	4,503,000		4,000,000
Gross Unrealized Gains	7,000		4,000
Gross Unrealized Losses	0		0
Fair Value	4,510,000		4,004,000
Municipal bonds
Schedule of Available-for-Sale Securities [Line Items]
Cost	636,000		4,564,000
Gross Unrealized Gains	5,000		17,000
Gross Unrealized Losses	0		(16,000)
Fair Value	641,000		4,565,000
Commercial paper
Schedule of Available-for-Sale Securities [Line Items]
Cost	3,791,000		6,269,000
Gross Unrealized Gains	7,000		4,000
Gross Unrealized Losses	0		0
Fair Value	3,798,000		6,273,000
U.S. Treasury notes
Schedule of Available-for-Sale Securities [Line Items]
Cost	23,814,000		23,820,000
Gross Unrealized Gains	3,000		1,000
Gross Unrealized Losses	(27,000)		(12,000)
Fair Value	23,790,000		23,809,000
U.S. Treasury bonds
Schedule of Available-for-Sale Securities [Line Items]
Cost	10,001,000		5,001,000
Gross Unrealized Gains	1,000		0
Gross Unrealized Losses	(3,000)		(2,000)
Fair Value	9,999,000		4,999,000
Corporate notes
Schedule of Available-for-Sale Securities [Line Items]
Cost	1,807,000		2,709,000
Gross Unrealized Gains	0		1,000
Gross Unrealized Losses	(1,000)		(2,000)
Fair Value	1,806,000		2,708,000
Corporate bonds
Schedule of Available-for-Sale Securities [Line Items]
Cost	5,145,000		6,395,000
Gross Unrealized Gains	20,000		3,000
Gross Unrealized Losses	(13,000)		(26,000)
Fair Value	$ 5,152,000		$ 6,372,000