DEBT - Summary of Liability Component of Convertible Notes (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Aug. 13, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Total	$ 94,661		$ 158,309
2.50% Convertible Notes due 2021 \| Convertible debt
Debt Instrument [Line Items]
Principal amount of the Notes	145,000		345,000
Unamortized discount of the liability component	(17,016)		(54,521)
Unamortized debt issuance costs	(817)		(2,681)
Total	127,166		$ 287,798
5.00% Convertible Notes due 2024 \| Convertible debt
Debt Instrument [Line Items]
Principal amount of the Notes	120,000
Unamortized discount of the liability component	(53,407)	$ (54,200)
Unamortized debt issuance costs	(2,836)
Total	$ 63,757

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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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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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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