MARKETABLE SECURITIES - Schedule of Marketable Securities (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2019	Jun. 30, 2018	Jun. 30, 2018
Debt Securities, Available-for-sale [Abstract]
Amortized cost	$ 381,209
Fair value	381,374
Marketable Securities [Abstract]
Marketable securities, amortized cost basis	382,295	$ 299,119	$ 299,119
Marketable securities, gross unrealized gains	168	0	0
Marketable securities, gross unrealized losses	(3)	(461)	(461)
Marketable securities, amortized cost	382,460	298,658	298,658
Short-term available-for-sale securities:
Marketable Securities [Abstract]
Marketable securities, amortized cost basis	326,363	298,200	298,200
Marketable securities, gross unrealized gains	92	0	0
Marketable securities, gross unrealized losses	(3)	(461)	(461)
Marketable securities, amortized cost	326,452	297,739	297,739
Short-term available-for-sale securities: \| U.S. treasury securities
Debt Securities, Available-for-sale [Abstract]
Amortized cost	123,149	297,965	297,965
Gross unrealized gains	0	0	0
Gross unrealized losses	(3)	(461)	(461)
Fair value	123,146	297,504	297,504
Short-term available-for-sale securities: \| Commercial paper
Debt Securities, Available-for-sale [Abstract]
Amortized cost	110,478
Gross unrealized gains	0
Gross unrealized losses	0
Fair value	110,478
Short-term available-for-sale securities: \| Corporate bonds
Debt Securities, Available-for-sale [Abstract]
Amortized cost	61,713
Gross unrealized gains	60
Gross unrealized losses	0
Fair value	61,773
Short-term available-for-sale securities: \| Asset-backed securities
Debt Securities, Available-for-sale [Abstract]
Amortized cost	30,727
Gross unrealized gains	32
Gross unrealized losses	0
Fair value	30,759
Short-term available-for-sale securities: \| Mutual fund securities
Equity Securities, FV-NI [Abstract]
Amortized cost	296	235	235
Gross unrealized gains	0		0
Gross unrealized losses	0		0
Fair value	296	235	235
Long-term available-for-sale securities:
Marketable Securities [Abstract]
Marketable securities, amortized cost basis	55,932	919	919
Marketable securities, gross unrealized gains	76	0	0
Marketable securities, gross unrealized losses	0	0	0
Marketable securities, amortized cost	56,008	919	919
Long-term available-for-sale securities: \| Corporate bonds
Debt Securities, Available-for-sale [Abstract]
Amortized cost	32,421
Gross unrealized gains	44
Gross unrealized losses	0
Fair value	32,465
Long-term available-for-sale securities: \| Asset-backed securities
Debt Securities, Available-for-sale [Abstract]
Amortized cost	22,721
Gross unrealized gains	32
Gross unrealized losses	0
Fair value	22,753
Long-term available-for-sale securities: \| Mutual fund securities
Equity Securities, FV-NI [Abstract]
Amortized cost	790	919	919
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Fair value	$ 790	$ 919	$ 919

Equity Securities, FV-NI [Abstract]

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