Nature of the Business - Additional Information (Detail) - USD ($)		1 Months Ended	3 Months Ended		6 Months Ended
	Aug. 03, 2018	Mar. 31, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Nature Of Business [Line Items]
Accumulated deficit			$ (242,207,000)		$ (242,207,000)		$ (216,808,000)
Cash, cash equivalents and short-term investments			60,800,000		60,800,000
Net loss			$ (15,478,000)	$ (16,958,000)	(26,504,000)	$ (32,054,000)
Cash in operations					$ 24,865,000	$ 27,691,000
Leerink Partners LLC [Member] | ATM Program [Member]
Nature Of Business [Line Items]
Common stock aggregate offering price		$ 50,000,000
Percentage of gross proceeds from common stock sold through sales agreement		3.00%
Leerink Partners LLC [Member] | ATM Program [Member] | Subsequent Event [Member]
Nature Of Business [Line Items]
Number of shares sold	2,110,818
Net proceeds from sales of common stock	$ 10,500,000