CASH, CASH EQUIVALENTS, RESTRICTED CASH AND MARKETABLE SECURITIES - Summary of Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Mar. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 119,965	$ 78,003
Unrealized Losses	(30)	(62)
Fair Value	119,935	77,941
U.S. government and municipal obligations
Debt Securities, Available-for-sale [Line Items]
Fair Value	62,473	42,186
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Fair Value	54,701	33,003
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Fair Value	2,761	2,752
Short-term marketable securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	119,965	78,003
Unrealized Losses	(30)	(62)
Fair Value	119,935	77,941
Short-term marketable securities \| U.S. government and municipal obligations
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	62,502	42,246
Unrealized Losses	(29)	(60)
Fair Value	62,473	42,186
Short-term marketable securities \| Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	54,701	33,003
Unrealized Losses	0	0
Fair Value	54,701	33,003
Short-term marketable securities \| Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,762	2,754
Unrealized Losses	(1)	(2)
Fair Value	2,761	2,752
Long-term marketable securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	0	0
Unrealized Losses	0	0
Fair Value	$ 0	$ 0

Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

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