INDEBTEDNESS - Carrying Amount of Equity Component of Convertible Notes (Details) - Convertible Debt - USD ($) $ in Thousands |
Sep. 30, 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 |