CONVERTIBLE NOTES PAYABLE - Composition of Notes (Details) - Senior Notes - Senior Notes Due 2025 - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Sep. 10, 2018
Debt Instrument [Line Items]
Interest rate percentage	2.50%		2.50%
Long-term debt, gross	$ 276,000	$ 276,000
Unamortized debt discount	(58,848)	(65,963)
Unamortized debt issuance costs	(4,501)	(5,176)
Long-term debt	$ 212,651	$ 204,861

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Amount, after accumulated amortization, of debt discount.

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Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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