Intangible Assets and Goodwill - Goodwill (Details) $ in Millions	9 Months Ended	12 Months Ended
Dec. 31, 2018 USD ($)	Mar. 31, 2018 USD ($) unit
Goodwill [Roll Forward]
Goodwill	$ 2,299.0
Goodwill	6,782.0	$ 2,299.0
Number of reporting units (in units) \| unit		2
Semiconductor products
Goodwill [Roll Forward]
Goodwill	2,279.8
Additions due to the acquisition of Microsemi	4,483.0
Goodwill	6,762.8	$ 2,279.8
Technology licensing
Goodwill [Roll Forward]
Goodwill	19.2
Additions due to the acquisition of Microsemi	0.0
Goodwill	$ 19.2	$ 19.2

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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=77989000&loc=SL49117168-202975

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=99380562&loc=d3e13770-109266

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Amount of increase in asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized resulting from a business combination.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=108376223&loc=d3e13816-109267

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- Definition

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+ References

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- Definition

Number of reporting units tested for impairment of goodwill. A reporting unit is an operating segment or one level below an operating segment.

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