CONVERTIBLE NOTES PAYABLE - Schedule of Interest Expense (Details) - Senior Notes - Senior Notes Due 2025 - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Debt Instrument [Line Items]
Contractual interest expense	$ 1,725	$ 1,725	$ 5,175	$ 5,175
Amortization of debt discount	2,417	2,239	7,115	6,591
Amortization of debt issuance costs	225	224	674	672
Total interest expense for the 2025 Notes	$ 4,367	$ 4,188	$ 12,964	$ 12,438

Amount of noncash expense included in interest expense to amortize debt discount and premium associated with the related debt instruments. Excludes amortization of financing costs. Alternate captions include noncash interest expense.

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