Long-Term Debt - Convertible Notes Interest Expense (Details) - Convertible Senior Notes 1.00 percent due 2020 - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Long-Term Debt
Cash interest per contractual coupon rate	$ 719	$ 719
Amortization of note discount	2,638	2,503
Amortization of debt issuance costs	186	166
Total interest expense related to Convertible Notes	$ 3,543	$ 3,388

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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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Amount of amortization expense attributable to debt issuance costs.

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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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