INDEBTEDNESS - Convertible senior notes (Details) - Convertible Senior Notes [Member] - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Dec. 31, 2014
Principal amount	$ 118,750	$ 118,750	$ 143,800
Unamortized debt discount	(2,601)	(5,648)	$ (33,600)
Deferred financing costs	(291)	(639)
Net carrying value	$ 115,858	$ 112,463

Including the current and noncurrent portions, carrying amount of debt identified as being convertible into another form of financial instrument (typically the entity's common stock) as of the balance sheet date, which originally required full repayment more than twelve months after issuance or greater than the normal operating cycle of the company.

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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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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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