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OVERVIEW, BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Additional Information (Details) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Mar. 31, 2019 USD ($) segment	Mar. 31, 2018 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)
Collaborative Arrangements and Non-collaborative Arrangements [Line Items]
Number of reportable segments \| segment			1
Prepaid expenses and other current assets	$ (34,769)		$ (34,769)		$ (6,851)
Cash, cash equivalents and marketable securities	479,100		479,100		413,400
Accumulated deficit	(1,139,674)		(1,139,674)		(1,066,007)
Net loss - basic and diluted	(37,494)	$ (22,851)	(73,667)	$ (94,898)
Net income (loss)	(37,494)	(22,851)	(73,667)	(94,898)
Cost of goods sold	1,042	0	2,023	0
Inventory	4,397		$ 4,397		$ 121
Customer Concentration Risk \| Novartis Pharmaceutical \| Accounts Receivable
Collaborative Arrangements and Non-collaborative Arrangements [Line Items]
Concentration risk, percentage			30.00%		52.00%
Customer Concentration Risk \| Loxo Oncology \| Accounts Receivable
Collaborative Arrangements and Non-collaborative Arrangements [Line Items]
Concentration risk, percentage			0.00%		14.00%
Customer Concentration Risk \| Pierre Fabre \| Accounts Receivable
Collaborative Arrangements and Non-collaborative Arrangements [Line Items]
Concentration risk, percentage			14.00%		13.00%
Customer Concentration Risk \| McKesson Corporation \| Accounts Receivable
Collaborative Arrangements and Non-collaborative Arrangements [Line Items]
Concentration risk, percentage			11.00%		0.00%
Accumulated Deficit
Collaborative Arrangements and Non-collaborative Arrangements [Line Items]
Net income (loss)	(37,494)	$ (22,851)	$ (73,667)	$ (94,898)	$ (147,300)	$ (116,800)	$ (92,800)
BRAFTOVI and MEKTOVI
Collaborative Arrangements and Non-collaborative Arrangements [Line Items]
Inventory	11,300		11,300
Pro Forma \| BRAFTOVI and MEKTOVI
Collaborative Arrangements and Non-collaborative Arrangements [Line Items]
Cost of goods sold	$ 3,900		$ 4,600
Reclassification
Collaborative Arrangements and Non-collaborative Arrangements [Line Items]
Prepaid expenses and other current assets					$ 100