Customer-Funded Research & Development (Details) - USD ($)	3 Months Ended	9 Months Ended
Jan. 28, 2017	Jan. 30, 2016	Jan. 28, 2017	Jan. 30, 2016
Customer-Funded Research & Development
Revenue from customer-funded R&D	$ 9,089,000	$ 8,877,000	$ 38,367,000	$ 35,638,000

The amount of compensation earned (contract income) under a research and development arrangement accounted for as a contract to perform research and development for others.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 730
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6420387&loc=d3e23199-108380

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 730
-SubTopic 20
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=6420387&loc=d3e23221-108380

Name:	us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersCompensationEarned
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration