Commitments (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Future minimum lease commitments under operating leases [Abstract]
2019	$ 26.0
2020	18.7
2021	13.3
2022	10.2
2023	1.7
Thereafter	0.0
Total minimum payments	69.9
Rental expense under operating lease	30.0	$ 35.4	$ 23.3
Property, Plant and Equipment Purchase Commitment
Purchase Commitment, Excluding Long-term Commitment [Line Items]
Purchase commitment, remaining minimum amount committed	144.4
Wafer Foundry Purchase Commitment
Purchase Commitment, Excluding Long-term Commitment [Line Items]
Purchase commitment, remaining minimum amount committed	$ 101.6