Co-Promotion Agreement - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Co Promotion Agreement [Line Items]
Accrued expenses and other current liabilities	$ 94,170,000	$ 84,171,000
Co-Promotion Agreement (the Agreement) | Kowa Pharmaceuticals America
Co Promotion Agreement [Line Items]
Agreement amended date	Jul. 25, 2017
Promotion fee as a percentage of gross margin		18.50%
Maximum co-promotion tail payments receive period	3 years
Co-promotion tail payments receivable		$ 17,800,000
Tail payments during the period	$ 0
Accrued co- promotion tail payments	16,900,000	$ 16,600,000
Accrued expenses and other current liabilities	$ 9,500,000