INDEBTEDNESS - Net Carrying Amount of Equity Component of Convertible Notes (Details) - Convertible Debt - USD ($) $ in Thousands	Mar. 31, 2020	Mar. 05, 2020	Dec. 31, 2019	May 25, 2018	Mar. 06, 2017
Debt Instrument [Line Items]
Amount allocated to conversion option	$ 346,340		$ 196,731
Less: allocated issuance costs	(4,547)		(2,339)
Equity component, net	341,793		194,392
2025 Notes
Debt Instrument [Line Items]
Amount allocated to conversion option	154,600
Less: allocated issuance costs	(2,342)
Equity component, net	152,258	$ 154,600
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	36,490		41,481
Less: allocated issuance costs	(974)		(1,108)
Equity component, net	$ 35,516		$ 40,373		$ 86,200