INDEBTEDNESS - Convertible senior notes - Notes and term loan (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
INDEBTEDNESS
Contractual coupon	$ 1,644	$ 1,752	$ 3,275	$ 3,476
Amortization of debt discount	1,534	1,760	3,047	3,497
Amortization of finance fees	360	371	720	741
Capitalized interest	(36)	(105)	(111)	(297)
Interest Expense, Debt	$ 3,502	$ 3,778	$ 6,931	$ 7,417

Amount of noncash expense included in interest expense to amortize debt discount and premium associated with the related debt instruments. Excludes amortization of financing costs. Alternate captions include noncash interest expense.

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Amount of amortization expense attributable to debt issuance costs.

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Amount of interest costs capitalized disclosed as an adjusting item to interest costs incurred.

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Amount of the cost of borrowed funds accounted for as interest expense for debt.

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Represents the portion of interest incurred in the period on debt arrangements that was charged against earnings, excluding amortization of debt discount (premium) and financing costs.

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