Stock Incentive Plans - Summary of Activity Under the 2008 Plan and the 2016 Plan (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Employee awards
Aggregate intrinsic value
Vested and expected to vest	$ 100		$ 100
The 2008 Plan and the 2016 Plan | Employee awards
Number of Options
Beginning balance (in shares)	6,323,596
Granted (in shares)	1,708,309
Exercised (in shares)	(5,128)
Forfeited (in shares)	(753,914)
Ending balance (in shares)	7,272,863	6,323,596
Vested (in shares)	1,822,514
Vested and expected to vest (in shares)	6,661,467
Weighted-average exercise price
Beginning balance (in dollars per share)	$ 4.91
Granted (in dollars per share)	2.30
Exercised (in dollars per share)	0.47
Forfeited (in dollars per share)	3.62
Ending balance (in dollars per share)	4.44	$ 4.91
Vested (in dollars per share)	8.39
Vested and expected to vest (in dollars per share)	$ 4.61
Weighted-average remaining contractual term
Outstanding, term	8 years 8 months 15 days	8 years 8 months 15 days
Vested, term	7 years 3 days
Vested and expected to vest, term	8 years 7 months 20 days
Aggregate intrinsic value
Outstanding	$ 1,728	$ 1,716
Vested	115
Vested and expected to vest	$ 1,541
The 2008 Plan and the 2016 Plan | Non‑employee awards
Number of Options
Beginning balance (in shares)	473,073
Granted (in shares)	0
Exercised (in shares)	0
Forfeited (in shares)	0
Ending balance (in shares)	473,073	473,073
Vested (in shares)	346,860
Vested and expected to vest (in shares)	473,073
Weighted-average exercise price
Beginning balance (in dollars per share)	$ 5.89
Granted (in dollars per share)	0
Exercised (in dollars per share)	0
Forfeited (in dollars per share)	0
Ending balance (in dollars per share)	5.89	$ 5.89
Vested (in dollars per share)	5.77
Vested and expected to vest (in dollars per share)	$ 5.89
Weighted-average remaining contractual term
Outstanding, term	5 years 11 months 23 days	6 years 2 months 23 days
Vested, term	4 years 11 months 23 days
Vested and expected to vest, term	5 years 11 months 23 days
Aggregate intrinsic value
Outstanding	$ 40	$ 38
Vested	22
Vested and expected to vest	$ 40