INDEBTEDNESS - Net Carrying Amount 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] | ||||
| Principal outstanding | $ 1,074,226 | $ 1,074,228 | ||
| Unamortized debt discount | (139,332) | (166,493) | ||
| Unamortized debt issuance costs | (6,025) | (8,040) | ||
| Net carrying value | 928,869 | 899,695 | ||
| 2023 Notes | ||||
| Debt Instrument [Line Items] | ||||
| Principal outstanding | 862,500 | 862,500 | ||
| Unamortized debt discount | (117,932) | (138,924) | ||
| Unamortized debt issuance costs | (3,647) | (5,054) | $ (5,600) | |
| Net carrying value | 740,921 | 718,522 | ||
| 2022 Notes | ||||
| Debt Instrument [Line Items] | ||||
| Principal outstanding | 211,726 | 211,728 | ||
| Unamortized debt discount | (21,400) | (27,569) | ||
| Unamortized debt issuance costs | (2,378) | (2,986) | $ (9,400) | |
| Net carrying value | $ 187,948 | $ 181,173 |