INDEBTEDNESS - Carrying Amount of Equity Component of Convertible Notes (Details) - Convertible Debt - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	May 25, 2018	Mar. 06, 2017
Debt Instrument [Line Items]
Amount allocated to conversion option	$ 196,731	$ 196,731
Less: allocated issuance costs	(2,339)	(2,339)
Equity component, net	194,392	194,392
2023 Notes
Debt Instrument [Line Items]
Amount allocated to conversion option	155,250	155,250
Less: allocated issuance costs	(1,231)	(1,231)
Equity component, net	154,019	154,019	$ 155,300
2022 Notes
Debt Instrument [Line Items]
Amount allocated to conversion option	41,481	41,481
Less: allocated issuance costs	(1,108)	(1,108)
Equity component, net	$ 40,373	$ 40,373		$ 86,200