CONVERTIBLE SENIOR NOTES AND CAPPED CALL - Carrying Amount of the Liability Component (Details) - Convertible Debt - Convertible Senior Notes - USD ($)	Mar. 31, 2019	Feb. 28, 2019
Debt Instrument [Line Items]
Principal	$ 287,500,000	$ 287,500,000.0
Unamortized debt discount	(70,876,000)
Unamortized issuance costs	(589,000)
Net carrying amount	$ 216,035,000

Face (par) amount of debt instrument at time of issuance.

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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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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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