Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands			1 Months Ended	3 Months Ended						9 Months Ended
	Jun. 21, 2019	Jun. 18, 2019	Feb. 28, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Nature Of Business And Basis Of Presentation [Line Items]
Entity incorporated year										Oct. 31, 2010
Entity incorporation, state or country code										DE
Percent of reduction in headcount of employees			30.00%
Aggregate net proceeds from offering of common stock										$ 60,527
Accumulated deficit				$ 440,871						440,871		$ 389,370
Net loss				(16,409)	$ (10,759)	$ (24,333)	$ (21,949)	$ (27,787)	$ (27,919)	(51,501)	$ (77,655)
Cash used in operations										(62,473)	$ (75,520)
Cash, cash equivalents and investments				$ 83,788						$ 83,788
Common Stock [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Common stock, shares issued					28,818,578
Underwritten Public Offering [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Aggregate net proceeds from offering of common stock	$ 4,551	$ 55,976
Underwritten Public Offering [Member] | Common Stock [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Common stock, shares issued	2,151,911	26,666,667
Common stock, share price	$ 2.25	$ 2.25
Underwritten Public Offering [Member] | Common Stock [Member] | Maximum [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Option to purchase additional shares of common stock		2,666,666