Stock-Based Compensation - Summary of Stock Option and Award Activity Excluding Inducement Grants (Detail) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Shares Available for Grant
Beginning balance (in shares)	55,799
Additional shares reserved (in shares)	419,056
Options granted (in shares)	(496,950)
Options canceled/forfeited/expired (in shares)	39,452
Ending balance (in shares)	17,357	55,799
Number of Shares Subject to Outstanding Options
Beginning balance (in shares)	1,296,157
Options granted (in shares)	496,950
Options canceled/forfeited/expired (in shares)	(39,452)
Ending balance (in shares)	1,753,655	1,296,157
Exercisable (in shares)	366,022
Vested or expected to vest (in shares)	1,627,577
Weighted-Average Exercise Price per Share
Beginning balance (in USD per share)	$ 5.75
Options granted (in USD per share)	3.27
Option canceled/forfeited/expired (in USD per share)	5.23
Ending balance (in USD per share)	5.06	$ 5.75
Exercisable (in USD per share)	8.55
Vested or expected to vest (in USD per share)	$ 5.15
Weighted-Average Remaining Contractual Term (In Years)
Weighted-average remaining contractual term	9 years 18 days	9 years 2 months 23 days
Exercisable	8 years 5 months 26 days
Vested or expected to vest	9 years 10 days
Aggregate Intrinsic Value
Balance	$ 197
Exercisable	1
Vested or expected to vest	$ 176