DEBT - Summary of Liability Component of Convertible Notes (Details) - Convertible debt - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
2.5% Convertible Notes debt 2021
Debt Instrument [Line Items]
Principal amount of the Notes	$ 42,465	$ 145,000
Unamortized discount of the liability component	(3,767)	(14,963)
Unamortized debt issuance costs	(184)	(725)
Total	38,514	129,312
5.0% Convertible Notes debt 2024
Debt Instrument [Line Items]
Principal amount of the Notes	34,522	120,000
Unamortized discount of the liability component	(14,373)	(51,701)
Unamortized debt issuance costs	(797)	(2,796)
Total	$ 19,352	$ 65,503

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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Amount, after accumulated amortization, of debt discount.

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Carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer), excluding current portion. Senior note holders are paid off in full before any payments are made to junior note holders.

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The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

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