Stock Incentive Plans - Summary of Activity Under the 2008 Plan and the 2016 Plan (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Nov. 20, 2019	Dec. 31, 2017
Employee awards
Aggregate intrinsic value
Vested and expected to vest	$ 100	$ 600		$ 2,600
Non‑employee awards
Number of Options
Vested and expected to vest (in shares)			141,328
Weighted-average exercise price
Granted (in dollars per share)	$ 4.35
The 2008 Plan and the 2016 Plan | Employee awards
Number of Options
Beginning balance (in shares)	3,681,575
Granted (in shares)	3,876,255
Exercised (in shares)	(125,600)
Forfeited (in shares)	(1,108,634)
Ending balance (in shares)	6,323,596	3,681,575
Vested (in shares)	1,485,228
Vested and expected to vest (in shares)	5,843,053
Weighted-average exercise price
Beginning balance (in dollars per share)	$ 9.49
Granted (in dollars per share)	2.01
Exercised (in dollars per share)	1.20
Forfeited (in dollars per share)	10.38
Ending balance (in dollars per share)	4.91	$ 9.49
Vested (in dollars per share)	10.02
Vested and expected to vest (in dollars per share)	$ 5.10
Weighted-average remaining contractual term
Outstanding, term	8 years 8 months 15 days	7 years 9 months 7 days
Vested, term	6 years 8 months 23 days
Vested and expected to vest, term	8 years 7 months 24 days
Aggregate intrinsic value
Outstanding	$ 1,716	$ 300
Vested	90
Vested and expected to vest	$ 1,500
The 2008 Plan and the 2016 Plan | Non‑employee awards
Number of Options
Beginning balance (in shares)	412,404
Granted (in shares)	73,489	0
Exercised (in shares)	0
Forfeited (in shares)	(12,820)
Ending balance (in shares)	473,073	412,404
Vested (in shares)	330,528
Vested and expected to vest (in shares)	473,073
Weighted-average exercise price
Beginning balance (in dollars per share)	$ 6.44
Granted (in dollars per share)	1.86
Exercised (in dollars per share)	0
Forfeited (in dollars per share)	0.47
Ending balance (in dollars per share)	5.89	$ 6.44
Vested (in dollars per share)	5.61
Vested and expected to vest (in dollars per share)	$ 5.89
Weighted-average remaining contractual term
Outstanding, term	6 years 2 months 23 days	6 years 5 months 1 day
Vested, term	5 years 21 days
Vested and expected to vest, term	6 years 2 months 23 days
Aggregate intrinsic value
Outstanding	$ 38	$ 28
Vested	21
Vested and expected to vest	$ 38