Debt - Total Interest Expense Recognized Related to the Convertible Notes (Details) - Convertible Debt - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Debt Instrument [Line Items]
Contractual interest expense	$ 2,734	$ 1,943	$ 5,468	$ 3,193
Amortization of debt issuance costs	347	321	685	596
Amortization of debt discount	3,313	2,716	6,550	4,644
Total interest expense	$ 6,394	$ 4,980	$ 12,703	$ 8,433

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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