CONVERTIBLE NOTES PAYABLE - Schedule of Interest Expense (Details) - Senior Notes - Senior Notes Due 2025 - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Debt Instrument [Line Items]
Contractual interest expense	$ 1,725	$ 1,725	$ 3,450	$ 3,450
Amortization of debt discount	2,371	2,197	4,698	4,352
Amortization of debt issuance costs	225	224	449	448
Total interest expense for the 2025 Notes	$ 4,321	$ 4,146	$ 8,597	$ 8,250

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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Amount of amortization expense attributable to debt issuance costs.

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Amount of the cost of borrowed funds accounted for as interest expense for debt.

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