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Co-Promotion Agreement - Additional Information (Detail) - USD ($) $ in Thousands | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2018 | Dec. 31, 2018 | Dec. 31, 2017
Co Promotion Agreement [Line Items] |  |  |
Accrued expenses and other current liabilities | $ 93,899 |  | $ 58,902
Co-Promotion Agreement (the Agreement) |  |  |
Co Promotion Agreement [Line Items] |  |  |
Promotion fee as a percentage of gross margin |  |  | 18.00%
Promotion fee as a percentage gross margin | amended, approximately eighteen percent (18%) in 2017, partially offset by certain other refinements |  |
Maximum co-promotion tail payments receive period | 3 years |  |
Accrued co- promotion tail payments | $ 10,800 |  | $ 0
Co-Promotion Agreement (the Agreement) | Scenario Forecast |  |  |
Co Promotion Agreement [Line Items] |  |  |
Promotion fee as a percentage of gross margin |  | 18.50% |
Co-Promotion Agreement (the Agreement) | Kowa Pharmaceuticals America |  |  |
Co Promotion Agreement [Line Items] |  |  |
Agreement amended date | Jul. 25, 2017 |  |
Accrued expenses and other current liabilities | $ 18,000 |  | $ 8,300

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