Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Entity incorporated year									Oct. 31, 2010
Entity incorporated state name									State of Delaware
Accumulated deficit	$ 263,571				$ 174,191				$ 263,571	$ 174,191
Net loss	(28,953)	$ (6,935)	$ (28,018)	$ (25,474)	$ (25,274)	$ (18,688)	$ (27,913)	$ (19,704)	(89,380)	(91,579)	$ (54,780)
Cash used in operations									(75,523)	$ 43,921	$ (40,844)
Cash, cash equivalents and investments	$ 149,983								$ 149,983