Long-term Debt - Interest Expense on Note (Details) - Senior notes - Convertible Senior Notes Due 2023 - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2018
Debt Instrument [Line Items]
Coupon interest	$ 190	$ 190
Amortization of debt discount	200	200
Amortization of debt issuance costs	45	45
Total interest expense recognized	$ 435	$ 435

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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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 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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