CASH, CASH EQUIVALENTS, AND INVESTMENTS (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Mar. 31, 2016
Schedule of Cash, Cash Equivalents and Investments [Line Items]
Cash and cash equivalents	$ 303,116	$ 301,970	$ 244,455	$ 235,266
Short-term investments (due in 1 year or less)	197,278	178,179
Long-term investments (due in 1 to 3 years)	105,251	127,176
Total cash, cash equivalents and investments measured at fair value, amortized cost	605,157	607,374
Total cash, cash equivalents and investments measured at fair value, gross unrealized gains	815	439
Total cash, cash equivalents and investments measured at fair value, gross unrealized losses	(327)	(488)
Total cash, cash equivalents and investments measured at fair value, fair value	605,645	607,325
Cash [Member]
Schedule of Cash, Cash Equivalents and Investments [Line Items]
Cash and cash equivalents	290,429	295,877
Level 1 [Member]
Schedule of Cash, Cash Equivalents and Investments [Line Items]
Cash and cash equivalents	0	0
Available-for-sale Securities, amortized cost basis	46,465	48,039
Available-for-sale Securities, gross unrealized gains	675	352
Available-for-sale Securities, gross unrealized losses	(93)	(100)
Available-for-sale Securities, fair value	47,047	48,291
Short-term investments (due in 1 year or less)	34,239	29,959
Long-term investments (due in 1 to 3 years)	12,808	18,332
Level 1 [Member] \| Mutual Funds [Member]
Schedule of Cash, Cash Equivalents and Investments [Line Items]
Cash and cash equivalents	0	0
Available-for-sale Securities, amortized cost basis	13,507	12,079
Available-for-sale Securities, gross unrealized gains	675	352
Available-for-sale Securities, gross unrealized losses	(27)	(32)
Available-for-sale Securities, fair value	14,155	12,399
Short-term investments (due in 1 year or less)	14,155	12,399
Long-term investments (due in 1 to 3 years)	0	0
Level 1 [Member] \| US Treasury Notes [Member]
Schedule of Cash, Cash Equivalents and Investments [Line Items]
Cash and cash equivalents	0	0
Available-for-sale Securities, amortized cost basis	32,958	35,960
Available-for-sale Securities, gross unrealized gains	0	0
Available-for-sale Securities, gross unrealized losses	(66)	(68)
Available-for-sale Securities, fair value	32,892	35,892
Short-term investments (due in 1 year or less)	20,084	17,560
Long-term investments (due in 1 to 3 years)	12,808	18,332
Level 2 [Member]
Schedule of Cash, Cash Equivalents and Investments [Line Items]
Cash and cash equivalents	12,687	6,093
Available-for-sale Securities, amortized cost basis	268,263	263,458
Available-for-sale Securities, gross unrealized gains	140	87
Available-for-sale Securities, gross unrealized losses	(234)	(388)
Available-for-sale Securities, fair value	268,169	263,157
Short-term investments (due in 1 year or less)	163,039	148,220
Long-term investments (due in 1 to 3 years)	92,443	108,844
Level 2 [Member] \| Government Agency Securities [Member]
Schedule of Cash, Cash Equivalents and Investments [Line Items]
Cash and cash equivalents	0	0
Available-for-sale Securities, amortized cost basis	47,851	54,415
Available-for-sale Securities, gross unrealized gains	0	20
Available-for-sale Securities, gross unrealized losses	(139)	(164)
Available-for-sale Securities, fair value	47,712	54,271
Short-term investments (due in 1 year or less)	26,870	15,309
Long-term investments (due in 1 to 3 years)	20,842	38,962
Level 2 [Member] \| Commercial Paper [Member]
Schedule of Cash, Cash Equivalents and Investments [Line Items]
Cash and cash equivalents	11,185	6,093
Available-for-sale Securities, amortized cost basis	52,099	47,152
Available-for-sale Securities, gross unrealized gains	0	0
Available-for-sale Securities, gross unrealized losses	0	0
Available-for-sale Securities, fair value	52,099	47,152
Short-term investments (due in 1 year or less)	40,914	41,059
Long-term investments (due in 1 to 3 years)	0	0
Level 2 [Member] \| Corporate Bonds [Member]
Schedule of Cash, Cash Equivalents and Investments [Line Items]
Cash and cash equivalents	0	0
Available-for-sale Securities, amortized cost basis	146,781	141,508
Available-for-sale Securities, gross unrealized gains	126	64
Available-for-sale Securities, gross unrealized losses	(95)	(224)
Available-for-sale Securities, fair value	146,812	141,348
Short-term investments (due in 1 year or less)	78,730	73,676
Long-term investments (due in 1 to 3 years)	68,082	67,672
Level 2 [Member] \| Certificates of Deposit [Member]
Schedule of Cash, Cash Equivalents and Investments [Line Items]
Cash and cash equivalents	1,502	0
Available-for-sale Securities, amortized cost basis	21,532	20,383
Available-for-sale Securities, gross unrealized gains	14	3
Available-for-sale Securities, gross unrealized losses	0	0
Available-for-sale Securities, fair value	21,546	20,386
Short-term investments (due in 1 year or less)	16,525	18,176
Long-term investments (due in 1 to 3 years)	$ 3,519	$ 2,210

Cash, Cash Equivalents, Short-term and Long-term Investments, Amortized Cost

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

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The total amount of investments that are intended to be held for an extended period of time (longer than one operating cycle).

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Amount of investments including trading securities, available-for-sale securities, held-to-maturity securities, and short-term investments classified as other and current.

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