COLLABORATION AND OTHER AGREEMENTS - Additional Information (Details) € in Millions, ¥ in Millions				1 Months Ended					3 Months Ended			9 Months Ended			12 Months Ended
	May 31, 2017 USD ($)	May 31, 2017 JPY (¥) milestone	Mar. 02, 2015	Dec. 31, 2018 USD ($)	Aug. 31, 2018 USD ($)	Apr. 30, 2018 USD ($) milestone	Dec. 31, 2015 EUR (€)	Dec. 31, 2015 USD ($)	Mar. 31, 2019 USD ($) milestone	Jun. 30, 2018	Mar. 31, 2018 USD ($)	Mar. 31, 2019 EUR (€)	Mar. 31, 2019 USD ($) milestone	Mar. 31, 2018 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($) milestone	Jul. 31, 2013 USD ($) milestone
Collaboration and Other Agreements, by Type [Line Items]
Revenue									$ 64,677,000		$ 66,367,000		$ 204,136,000	$ 138,331,000
Foreign currency exchange rate translation									110.90				110.90
Pierre Fabre
Collaboration and Other Agreements, by Type [Line Items]
Upfront cash payment															$ 30,000,000
Milestone earned									$ 15,000,000
Royalty revenue threshold | €							€ 50.0					€ 100.0
Pierre Fabre | Below revenue threshold
Collaboration and Other Agreements, by Type [Line Items]
Royalty percentage							20.00%	20.00%
Pierre Fabre | Above revenue threshold
Collaboration and Other Agreements, by Type [Line Items]
Royalty percentage							35.00%	35.00%
Pierre Fabre | Maximum | Above revenue threshold, adjusted for incurred costs
Collaboration and Other Agreements, by Type [Line Items]
Royalty percentage							50.00%	50.00%
Royalty revenue threshold								$ 15,000,000
Pierre Fabre | Commercial milestones
Collaboration and Other Agreements, by Type [Line Items]
Additional potential milestone payments																$ 390,000,000
Number of milestones | milestone																7
Ono
Collaboration and Other Agreements, by Type [Line Items]
Upfront cash payment	$ 31,200,000	¥ 3,500.0
Royalty revenue threshold | ¥		¥ 10,000.0
Ono | Below revenue threshold
Collaboration and Other Agreements, by Type [Line Items]
Royalty percentage	22.00%	22.00%
Ono | Above revenue threshold
Collaboration and Other Agreements, by Type [Line Items]
Royalty percentage	25.00%	25.00%
Ono | Regulatory milestones
Collaboration and Other Agreements, by Type [Line Items]
Additional potential milestone payments | ¥		¥ 5,000.0
Number of milestones | milestone		8
Ono | Developmental milestones
Collaboration and Other Agreements, by Type [Line Items]
Additional potential milestone payments | ¥		¥ 500.0
Number of milestones | milestone		1
Ono | Commercial milestones
Collaboration and Other Agreements, by Type [Line Items]
Additional potential milestone payments | ¥		¥ 10,500.0
Number of milestones | milestone		5
Loxo Oncology
Collaboration and Other Agreements, by Type [Line Items]
Milestone earned				$ 20,000,000	$ 4,000,000
Additional potential milestone payments									20,000,000				$ 20,000,000
Loxo Oncology | Other current assets
Collaboration and Other Agreements, by Type [Line Items]
Additional potential milestone payments									10,000,000				10,000,000
Loxo Oncology | Other noncurrent assets
Collaboration and Other Agreements, by Type [Line Items]
Additional potential milestone payments									$ 10,000,000				$ 10,000,000
Loxo Oncology | Developmental milestones
Collaboration and Other Agreements, by Type [Line Items]
Additional potential milestone payments																	$ 7,000,000
Number of milestones | milestone																	2
Loxo Oncology | Commercial milestones
Collaboration and Other Agreements, by Type [Line Items]
Revenue				$ 40,000,000
Number of additional milestone payments | milestone									2				2
Additional potential milestone payments									$ 10,000,000				$ 10,000,000				$ 595,000,000
Number of milestones | milestone																	20
Mirati
Collaboration and Other Agreements, by Type [Line Items]
Upfront cash payment						$ 2,000,000
Deferred revenue recognition period										2 years
Mirati | Developmental milestones
Collaboration and Other Agreements, by Type [Line Items]
Additional potential milestone payments						$ 17,300,000
Number of milestones | milestone						6
Mirati | Commercial milestones
Collaboration and Other Agreements, by Type [Line Items]
Additional potential milestone payments						$ 674,000,000
Number of milestones | milestone						14
Other Collaboration Arrangements | Regulatory milestones
Collaboration and Other Agreements, by Type [Line Items]
Potential milestone payments									73,000,000				73,000,000
Other Collaboration Arrangements | Developmental milestones
Collaboration and Other Agreements, by Type [Line Items]
Potential milestone payments									48,500,000				48,500,000
Other Collaboration Arrangements | Revenue based on sales milestone
Collaboration and Other Agreements, by Type [Line Items]
Potential milestone payments									$ 159,500,000				$ 159,500,000
Novartis Agreements
Collaboration and Other Agreements, by Type [Line Items]
Royalty percentage			2.00%