INDEBTEDNESS - Net Carrying Amount of Convertible Notes (Details) - Convertible Debt - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	May 25, 2018	Mar. 06, 2017
Debt Instrument [Line Items]
Principal outstanding	$ 1,074,226	$ 1,074,228
Unamortized debt discount	(129,830)	(166,493)
Unamortized debt issuance costs	(5,564)	(8,040)
Net carrying value	938,832	899,695
2023 Notes
Debt Instrument [Line Items]
Principal outstanding	862,500	862,500
Unamortized debt discount	(110,518)	(138,924)
Unamortized debt issuance costs	(3,418)	(5,054)	$ (5,600)
Net carrying value	748,564	718,522
2022 Notes
Debt Instrument [Line Items]
Principal outstanding	211,726	211,728
Unamortized debt discount	(19,312)	(27,569)
Unamortized debt issuance costs	(2,146)	(2,986)		$ (9,400)
Net carrying value	$ 190,268	$ 181,173

Debt Issuance Costs, Amount Allocated to Liability Component of Convertible Debt

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- Definition

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.

+ References

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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 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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