Research and Development Agreements - Schedule of Impact of Adoption on Condensed Consolidated Statement of Operations and Comprehensive (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenue Recognition [Line Items]
Research and development revenue	$ 944	$ 0	$ 1,175	$ 1,760
Total revenue	1,367	42	1,849	1,927
Net loss	$ 9,695	$ 4,064	$ 18,409	$ 5,258
Basic and diluted	$ 0.36	$ 20.76	$ 0.68	$ 28.32
ASC 2014-09 | Effect of Adoption of ASC 606
Revenue Recognition [Line Items]
Research and development revenue	$ (11)		$ (79)
Total revenue	(11)		(79)
Net loss	$ 11		$ 79
Basic and diluted	$ (0.00)		$ (0.00)
ASC 2014-09 | Balance without Adoption of ASC 606
Revenue Recognition [Line Items]
Research and development revenue	$ 933		$ 1,096
Total revenue	1,356		1,770
Net loss	$ 9,706		$ 18,488
Basic and diluted	$ 0.36		$ 0.68