Liability Related to Sale of Future Royalties - Additional Information (Detail) - USD ($) $ in Thousands		3 Months Ended				70 Months Ended
	Feb. 24, 2012	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2017
Liability Related to Sale of Future Royalties [Line Items]
Non-cash royalty revenue related to sale of future royalties		$ 8,230		$ 6,920
Non-cash interest expense on liability related to sale of future royalties		$ 6,065		$ 5,019
Annual interest rate			18.70%		17.60%	17.00%
Prospective interest rate			29.00%		21.00%
Purchase and Sale Agreement with RPI [Member]
Liability Related to Sale of Future Royalties [Line Items]
Proceeds from sale of royalty rights	$ 124,000
Transaction costs related to sale of potential future royalties	$ 4,400