FAIR VALUE MEASUREMENTS - Narrative (Details) - USD ($) $ in Millions |
Sep. 30, 2019 |
Dec. 31, 2018 |
|---|---|---|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ||
| Cash and Cash Equivalents, Fair Value Disclosure | $ 108.0 | |
| Fair Value, Inputs, Level 2 | Convertible Debt | Convertible Senior Notes Due 2023 | ||
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ||
| Fair value of convertible debt | $ 152.1 | $ 130.3 |