CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY (USD $) In Thousands, except Share data, unless otherwise specified	Total	Senior Subordinated Convertible Debenture Due 2025 [Member]	Junior Subordinated Convertible Debentures Due2037 Member	SMSC Acquisition [Member]	Supertex Inc. [Member]	Common Stock and Additional Paid-in Capital	Common Stock and Additional Paid-in Capital Senior Subordinated Convertible Debenture Due 2025 [Member]	Common Stock and Additional Paid-in Capital Junior Subordinated Convertible Debentures Due2037 Member	Common Stock and Additional Paid-in Capital SMSC Acquisition [Member]	Common Stock and Additional Paid-in Capital Supertex Inc. [Member]	Common Stock Held in Treasury	Accumulated Other Comprehensive Income (Loss)	Retained Earnings	Parent [Member]	Parent [Member] Senior Subordinated Convertible Debenture Due 2025 [Member]	Parent [Member] Junior Subordinated Convertible Debentures Due2037 Member	Parent [Member] SMSC Acquisition [Member]	Parent [Member] Supertex Inc. [Member]	Noncontrolling Interest [Member]
Balance at Mar. 31, 2012	$ 1,990,673					$ 1,269,100					$ (780,893)	$ 3,101	$ 1,499,365	$ 1,990,673					$ 0
Balance, treasury stock, (shares) at Mar. 31, 2012											25,639,000
Balance, common stock shares issued (in shares) at Mar. 31, 2012						218,790,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	127,389												127,389	127,389
Other comprehensive income (loss)	3,834											3,834		3,834
Proceeds from sales of common stock through employee equity incentive plans (shares)						3,799,000
Proceeds from sales of common stock through employee equity incentive plans (amount)	51,365					51,365								51,365
Restricted stock unit and stock appreciation right withholdings (shares)						(477,000)
Restricted stock unit and stock appreciation right withholdings (amount)	(15,670)					(15,670)								(15,670)
Treasury stock used for new issuances (shares)						(3,322,000)					(3,322,000)
Treasury stock used for new issuances (amount)						(98,673)					98,673
Tax benefit (shortfall) from equity incentive plans	(9,896)					(9,896)								(9,896)
Share-based compensation	52,667					52,667								52,667
Non-cash consideration - business acquisitions				6,930					6,930								6,930
Cash dividend	(273,822)												(273,822)	(273,822)
Balance at Mar. 31, 2013	1,933,470					1,255,823					(682,220)	6,935	1,352,932	1,933,470					0
Balance, common stock shares issued (in shares) at Mar. 31, 2013						218,790,000
Balance, treasury stock, (shares) at Mar. 31, 2013											22,317,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	395,281												395,281	395,281
Other comprehensive income (loss)	(5,884)											(5,884)		(5,884)
Proceeds from sales of common stock through employee equity incentive plans (shares)						4,161,000
Proceeds from sales of common stock through employee equity incentive plans (amount)	60,086					60,086								60,086
Restricted stock unit and stock appreciation right withholdings (shares)						(631,000)
Restricted stock unit and stock appreciation right withholdings (amount)	(22,640)					(22,640)								(22,640)
Treasury stock used for new issuances (shares)						(3,530,000)					(3,530,000)
Treasury stock used for new issuances (amount)						(104,838)					104,838
Tax benefit (shortfall) from equity incentive plans	1,411					1,411								1,411
Share-based compensation	54,941					54,941								54,941
Cash dividend	(281,204)												(281,204)	(281,204)
Balance at Mar. 31, 2014	2,135,461					1,244,783					(577,382)	1,051	1,467,009	2,135,461					0
Balance, common stock shares issued (in shares) at Mar. 31, 2014	218,789,994					218,790,000
Balance, treasury stock, (shares) at Mar. 31, 2014	200,002,736										18,787,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Additions due to acquisition of controlling interest in ISSC	52,467																		52,467
Net income	365,325												369,009	369,009					(3,684)
Other comprehensive income (loss)	9,750											10,616		10,616					(866)
Other	240																		240
Purchase of additional interests	(32,095)					345						(591)		(246)					(31,849)
Proceeds from sales of common stock through employee equity incentive plans (shares)						2,503,000
Proceeds from sales of common stock through employee equity incentive plans (amount)	34,433					34,369								34,369					64
Restricted stock unit and stock appreciation right withholdings (shares)						(426,000)
Restricted stock unit and stock appreciation right withholdings (amount)	(19,504)					(19,504)								(19,504)
Treasury stock used for new issuances (shares)						(2,077,000)					(2,077,000)
Treasury stock used for new issuances (amount)						(61,703)					61,703
Tax benefit (shortfall) from equity incentive plans	1,220					1,220								1,220
Share-based compensation	56,687					56,687								56,687
Non-cash consideration - business acquisitions					1,622					1,622								1,622
Convertible Debt - retirement of 2037 debentures			(606,926)					(606,926)								(606,926)
Convertible Debt - issuance of 2025 debentures		348,824					348,824								348,824
Cash dividend	(286,478)												(286,478)	(286,478)
Balance at Mar. 31, 2015	$ 2,061,026					$ 999,717					$ (515,679)	$ 11,076	$ 1,549,540	$ 2,044,654					$ 16,372
Balance, common stock shares issued (in shares) at Mar. 31, 2015	218,789,994					218,790,000
Balance, treasury stock, (shares) at Mar. 31, 2015	202,080,306										16,710,000