Intangible Assets and Goodwill - Goodwill by Reporting Segment (Details) $ in Millions	9 Months Ended	12 Months Ended
	Dec. 31, 2019 USD ($)	Mar. 31, 2019 USD ($) unit
Goodwill By Reporting Segment [Line Items]
Number of reporting units (in units) | unit		2
Goodwill [Roll Forward]
Goodwill opening balance	$ 6,663.9
Goodwill closing balance	6,664.8	$ 6,663.9
Semiconductor Products Reporting Unit
Goodwill [Roll Forward]
Goodwill opening balance	6,644.7
Additions	0.9
Goodwill closing balance	6,645.6	6,644.7
Technology Licensing Reporting Unit
Goodwill [Roll Forward]
Goodwill opening balance	19.2
Additions	0.0
Goodwill closing balance	$ 19.2	$ 19.2