Convertible Senior Notes - Summary of Total Interest Expense Recognized Related To Convertible Senior Notes (Details) - Convertible Senior Notes - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2018
Debt Instrument [Line Items]
Amortization of debt discount and transaction costs	$ 1,160	$ 3,435
Total interest expense	1,592	4,736
1.50% Coupon
Debt Instrument [Line Items]
Interest expense	$ 432	$ 1,301

Amount of amortization expense attributable to debt discount (premium) and 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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