INCOME TAXES (Details) - USD ($)		9 Months Ended	12 Months Ended
		Dec. 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Current:
Federal			$ 82,523,000	$ 10,591,000	$ 15,702,000
State			4,274,000	457,000	1,934,000
Foreign			6,860,000	7,731,000	4,644,000
Total current provision for income taxes			93,657,000	18,779,000	22,280,000
Deferred:
Federal			9,002,000	1,022,000	(7,767,000)
State			(1,585,000)	(117,000)	(1,103,000)
Foreign			22,000	(618,000)	374,000
Total deferred benefit for income taxes			7,439,000	287,000	(8,496,000)
Income tax expense			101,096,000	19,066,000	13,784,000
Components of income before income taxes
United States			17,654,000	43,377,000	42,184,000
Foreign			82,573,000	58,288,000	39,992,000
Income before income taxes			100,227,000	101,665,000	82,176,000
Reconciliation between statutory federal income taxes and income tax expense
Tax expense at statutory rate			31,631,000	35,583,000	28,762,000
Foreign operations taxed at different rates			(17,970,000)	(13,183,000)	(9,478,000)
State taxes, net of federal benefit			2,689,000	340,000	831,000
Research and development credit			(2,023,000)	(3,119,000)	(3,133,000)
Unwind of stock based compensation cost sharing			0	0	(2,855,000)
Impact of Tax Act			87,790,000	0	0
Stock based compensation			(1,771,000)	(365,000)	(487,000)
Other, net			750,000	(190,000)	144,000
Income tax expense			101,096,000	19,066,000	13,784,000
Components of Deferred Tax Assets
Accruals and other reserves			4,809,000	8,526,000
Deferred Compensation			3,467,000	4,859,000
Net operating loss carry forward			1,540,000	2,221,000
Stock compensation			8,384,000	12,821,000
Tax credits			6,504,000	4,530,000
Other deferred tax assets			1,070,000	1,711,000
Valuation allowance			(2,514,000)	(2,209,000)
Total deferred tax assets			23,260,000	32,459,000
Components of Deferred Tax Liabilities
Deferred gains on sales of properties			(1,160,000)	(1,771,000)
Unremitted earnings of certain subsidiaries			(1,976,000)	0
Fixed asset depreciation			(4,150,000)	(7,351,000)
Total deferred tax liabilities			(7,286,000)	(9,122,000)
Net deferred tax assets	[1]		15,974,000	23,337,000
Increase in valuation allowance			300,000
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at beginning of period		$ 12,854,000	12,854,000	12,692,000	12,821,000
Increase (decrease) of unrecognized tax benefits related to prior fiscal years			(1,310,000)	(2,000)	(598,000)
Increase of unrecognized tax benefits related to the current year			3,085,000	2,195,000	2,252,000
Reductions to unrecognized tax benefits related to settlements with taxing authorities			(115,000)	0	(149,000)
Reductions to unrecognized tax benefits related to lapse of applicable statute of limitations			(1,902,000)	(2,031,000)	(1,634,000)
Balance at end of period			12,612,000	12,854,000	$ 12,692,000
Tax penalties accrued			0	0
Interest related to unrecognized tax benefits			1,400,000	$ 1,700,000
Toll charge			79,700,000
Reduction in deferred tax assets			(4,600,000)
Undistributed foreign earnings			$ 5,000,000
Blended tax rate			31.56%
Transition tax, period under review		30 years
Excess tax benefits			$ 2,700,000
California Franchise Tax Board [Member]
Components of Deferred Tax Liabilities
Research and development credit carryforwards			10,400,000
State Administration of Taxation, China [Member]
Components of Deferred Tax Assets
Net operating loss carry forward			$ 4,300,000

[1] The Company's deferred tax assets for the fiscal year ending March 31, 2017 and March 31, 2018, are included as a component of other assets on the consolidated balance sheets.