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Fair Value Measurements and Short-Term Investments - Contingent Royalty Obligation (Details)
$ in Thousands
3 Months Ended
Mar. 31, 2019
USD ($)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]  
Beginning balance $ 50,200
FDA milestone payment (9,213)
Change in fair value recorded in operating expenses 4,913
Ending balance $ 45,900