Goodwill by Reporting Segment (Details) - USD ($) $ in Thousands	12 Months Ended
Mar. 31, 2016	Mar. 31, 2015
Goodwill [Roll Forward]
Goodwill, beginning balance	$ 571,271
Goodwill, ending balance	1,012,652	$ 571,271
Supertex Inc. [Member]
Goodwill [Roll Forward]
Goodwill, beginning balance	143,160
Goodwill, ending balance		143,160
Micrel Incorporated [Member]
Goodwill [Roll Forward]
Goodwill, ending balance	440,992
Semiconductor products [Member]
Goodwill [Roll Forward]
Goodwill, beginning balance	552,071	256,897
Goodwill, ending balance	993,452	552,071
Semiconductor products [Member] \| Other business acquisitions [Member]
Goodwill [Roll Forward]
Purchase accounting adjustments		624
Semiconductor products [Member] \| Supertex Inc. [Member]
Goodwill [Roll Forward]
Additions due to business combination		143,160
Semiconductor products [Member] \| ISSC Technologies Corporation [Member]
Goodwill [Roll Forward]
Additions due to business combination		154,399
Purchase accounting adjustments	389
Foreign currency translation adjustments		(3,009)
Semiconductor products [Member] \| Micrel Incorporated [Member]
Goodwill [Roll Forward]
Additions due to business combination	440,992
Technology licensing [Member]
Goodwill [Roll Forward]
Goodwill, beginning balance	19,200	19,200
Goodwill, ending balance	19,200	19,200
Technology licensing [Member] \| Other business acquisitions [Member]
Goodwill [Roll Forward]
Purchase accounting adjustments		0
Technology licensing [Member] \| Supertex Inc. [Member]
Goodwill [Roll Forward]
Additions due to business combination		0
Technology licensing [Member] \| ISSC Technologies Corporation [Member]
Goodwill [Roll Forward]
Additions due to business combination		0
Purchase accounting adjustments	0
Foreign currency translation adjustments		$ 0
Technology licensing [Member] \| Micrel Incorporated [Member]
Goodwill [Roll Forward]
Additions due to business combination	$ 0

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Amount of increase (decrease) from foreign currency translation adjustments of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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