Components of deferred tax assets and liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2014
Income Tax Disclosure [Abstract]
Excess Tax Benefit from Share-based Compensation, Operating Activities	$ 758	$ 1,216	$ 1,411
Components of Deferred Tax Assets [Abstract]
Deferred intercompany profit	12,642	10,865
Deferred income on shipments to distributors	34,830	34,493
Inventory valuation	12,082	9,605
Net operating loss carryforward	63,209	105,756
Capital loss carryforward	5,707	4,582
Share-based compensation	31,410	26,780
Income tax credits	100,294	115,893
Property, plant and equipment, principally due to differences in depreciation	16,262	2,236
Accrued expenses and other	7,559	671
Gross deferred tax assets	283,995	310,881
Valuation allowances	(161,834)	(116,482)
Deferred tax assets, net of valuation allowances	122,161	194,399
Components of Deferred Tax Liabilities [Abstract]
Convertible debentures	(496,626)	(493,897)
Other	(9,922)	(10,649)
Deferred Tax Liabilities, Gross	506,548	504,546
Net deferred tax liability	(384,387)	(310,147)
Current deferred tax assets	0	71,045
Non-current deferred tax assets	14,831	0
Non-current deferred tax liability		$ (381,192)
Deferred Income Tax Liabilities, Net	(399,218)
Unrecorded Tax Benefits Related to Share-Based Compensation	$ 47,300