Components of deferred tax assets and liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2018	Mar. 31, 2017
Components of Deferred Tax Assets [Abstract]
Deferred income on shipments to distributors	$ 39.1	$ 55.7
Inventory valuation	10.7	14.6
Net operating loss carryforward	101.1	91.6
Capital loss carryforward	10.6	12.9
Share-based compensation	31.4	42.5
Income tax credits	178.4	243.0
Property, plant and equipment	25.7	59.7
Accrued expenses and other	91.2	110.4
Gross deferred tax assets	488.2	630.4
Valuation allowances	(204.5)	(210.1)
Deferred tax assets, net of valuation allowances	283.7	420.3
Components of Deferred Tax Liabilities [Abstract]
Convertible debt	(304.4)	(606.7)
Intangible assets	(66.6)	(147.5)
Other	(18.3)	(6.3)
Deferred Tax Liabilities, Gross	(389.3)	(760.5)
Net deferred tax liability	(105.6)	(340.2)
Non-current deferred tax assets	100.2	68.9
Non-current deferred tax liability	$ (205.8)	$ (409.1)