Fair Value of Financial Assets and Liabilities - Summary of Short-Term Investments (Details) - USD ($) $ in Thousands	12 Months Ended
Apr. 30, 2017	Apr. 30, 2016
Schedule of Available-for-Sale Securities [Line Items]
Cost	$ 38,718	$ 50,692
Gross Unrealized Gains	0	5
Gross Unrealized Losses	(29)	(15)
Fair Value	38,689	50,682
Impairment Charges	0	0
Certificates of deposit
Schedule of Available-for-Sale Securities [Line Items]
Cost	1,930	7,090
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(1)
Fair Value	1,930	7,089
Commercial paper
Schedule of Available-for-Sale Securities [Line Items]
Cost	4,206	4,043
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	4,206	4,043
U.S. Treasury securities
Schedule of Available-for-Sale Securities [Line Items]
Cost	8,705	8,764
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(5)	0
Fair Value	8,700	8,764
US government agencies debt securities
Schedule of Available-for-Sale Securities [Line Items]
Cost	16,733	24,841
Gross Unrealized Gains	0	4
Gross Unrealized Losses	(15)	(9)
Fair Value	16,718	24,836
Corporate debt securities
Schedule of Available-for-Sale Securities [Line Items]
Cost	7,144	5,954
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(9)	(5)
Fair Value	$ 7,135	$ 5,950

This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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