DEBT - Schedule of Components of Unsecured Long-Term Debt Outstanding (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Dec. 20, 2018	Sep. 30, 2018
Debt
Face amount	$ 500,000			$ 500,000
Unamortized discount and debt issuance cost	(8,349)			(6,032)
Book value	491,651			493,968
Long-term debt due within one year, face amount	0			0
Unamortized discount and debt issuance cost	0			0
Long term debt due within one year, book value	0			0
Unamortized debt issuance cost	(8,349)			(6,032)
Long-term debt	491,651			493,968
HPIDC 2025 Notes
Debt
Face amount	500,000			500,000
Unamortized discount and debt issuance cost	(8,349)			(6,032)
Book value	$ 491,651	$ 12,900	$ 12,900	$ 493,968

Long-term Debt, Current Maturities, Gross

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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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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 issuance costs classified as current. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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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 accumulated amortization, of debt issuance costs classified as noncurrent. 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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