Intangible Assets, by Major Class (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Finite-Lived Intangible Assets [Line Items]
Gross Amount	$ 2,683.1	$ 2,706.5
Accumulated Amortization	(1,021.1)	(558.4)
Net Amount	1,662.0	2,148.1
In-Process Technology Reaching Technological Feasibility And Reclassified	26.2
Projected Amortization Expense
2019	362.8
2020	314.6
2021	259.3
2022	192.5
2023	143.7
Amortization of acquired intangible assets	492.2	346.3	$ 179.3
Impairment of intangible assets	0.5	11.9	0.6
Cost of Sales
Projected Amortization Expense
Amortization of acquired intangible assets	6.1	4.0	3.6
Operating Expense
Projected Amortization Expense
Amortization of acquired intangible assets	$ 486.1	342.3	$ 175.7
Minimum
Finite-Lived Intangible Assets [Line Items]
Useful life (in years)	1 year
Maximum
Finite-Lived Intangible Assets [Line Items]
Useful life (in years)	15 years
Core and developed technology
Finite-Lived Intangible Assets [Line Items]
Gross Amount	$ 1,952.3	1,932.3
Accumulated Amortization	(644.4)	(419.5)
Net Amount	1,307.9	1,512.8
Purchased intangible assets	$ 7.1
Acquired finite-lived intangible assets, weighted average useful life (in years)	9 years
Customer-related
Finite-Lived Intangible Assets [Line Items]
Gross Amount	$ 716.9	716.9
Accumulated Amortization	(375.9)	(123.6)
Net Amount	341.0	593.3
Trademarks and trade names
Finite-Lived Intangible Assets [Line Items]
Gross Amount		11.7
Accumulated Amortization		(9.6)
Net Amount		2.1
In-process research and development
Finite-Lived Intangible Assets [Line Items]
Gross Amount	12.1	38.5
Accumulated Amortization	0.0	0.0
Net Amount	12.1	38.5
Distribution rights
Finite-Lived Intangible Assets [Line Items]
Gross Amount	0.3	5.6
Accumulated Amortization	(0.1)	(5.3)
Net Amount	0.2	0.3
Other intangible assets
Finite-Lived Intangible Assets [Line Items]
Gross Amount	1.5	1.5
Accumulated Amortization	(0.7)	(0.4)
Net Amount	$ 0.8	$ 1.1