Intangible Assets and Goodwill - Goodwill (Details) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Mar. 31, 2018 USD ($) unit
Goodwill [Roll Forward]
Goodwill			$ 2,299.0
Goodwill	$ 6,787.5		6,787.5		$ 2,299.0
Amortization of intangible assets	173.8	$ 122.7	308.6	$ 245.5
Impairment of intangible assets	1.2		3.1	0.2
Number of reporting units (in units) | unit					2
Semiconductor products
Goodwill [Roll Forward]
Goodwill			2,279.8
Additions due to the acquisition of Microsemi			4,488.5
Goodwill	6,768.3		6,768.3		$ 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		$ 19.2
Cost of sales
Goodwill [Roll Forward]
Amortization of intangible assets	2.1	3.1	1.9	3.9
Operating expense
Goodwill [Roll Forward]
Amortization of intangible assets	$ 171.7	$ 305.5	$ 120.8	$ 241.6