CASH, CASH EQUIVALENTS, RESTRICTED CASH AND MARKETABLE SECURITIES - Summary of Marketable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Mar. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 106,761	$ 78,003
Unrealized (Losses) Gains	(6)	(62)
Fair Value	106,755	77,941
U.S. government and municipal obligations
Debt Securities, Available-for-sale [Line Items]
Fair Value	53,965	42,186
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Fair Value	50,783	33,003
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Fair Value	2,007	2,752
Short-term marketable securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	105,760	78,003
Unrealized (Losses) Gains	(7)	(62)
Fair Value	105,753	77,941
Short-term marketable securities \| U.S. government and municipal obligations
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	53,972	42,246
Unrealized (Losses) Gains	(7)	(60)
Fair Value	53,965	42,186
Short-term marketable securities \| Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	50,783	33,003
Unrealized (Losses) Gains	0	0
Fair Value	50,783	33,003
Short-term marketable securities \| Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,005	2,754
Unrealized (Losses) Gains	0	(2)
Fair Value	1,005	2,752
Long-term marketable securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,001	0
Unrealized (Losses) Gains	1	0
Fair Value	1,002	$ 0
Long-term marketable securities \| Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,001
Unrealized (Losses) Gains	1
Fair Value	$ 1,002

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

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

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

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Amount, before tax, of unrealized gain (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).

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