CONVERTIBLE SENIOR NOTES AND CAPPED CALL - Carrying Amount of the Liability Component (Details) - Convertible Debt - Convertible Senior Notes - USD ($)	Jun. 30, 2019	Mar. 31, 2019	Feb. 28, 2019
Debt Instrument [Line Items]
Principal	$ 287,500,000	$ 287,500,000	$ 287,500,000
Unamortized debt discount	(67,730,000)	(70,876,000)
Unamortized issuance costs	(562,000)	(589,000)
Net carrying amount	$ 219,208,000	$ 216,035,000