Stock-Based Compensation - Stock options (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended			24 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2019
Options
Outstanding balance at the beginning of the period (in shares)	2,761,912	2,257,621		2,257,621
Granted (in shares)	1,481,975	599,050
Exercised (in shares)		2,478
Forfeited (in shares)	(219,321)	(92,281)
Outstanding balance at the end of the period (in shares)	4,024,566	2,761,912	2,257,621	4,024,566
Options exercisable at the end of the period (in shares)	2,398,685			2,398,685
Options vested and expected to vest at end of period (in shares)	4,024,566			4,024,566
Options unvested at end of period (in shares)	90,500			90,500
Weighted Average Exercise Price Per Share
Outstanding balance at the beginning of the period (in dollars per share)	$ 11.30	$ 12.38		$ 12.38
Granted (in dollars per share)	1.48	6.68
Exercised (in dollars per share)		5.56
Forfeited (in dollars per share)	15.14	7.89
Outstanding balance at the end of the period (in dollars per share)	7.48	$ 11.30	$ 12.38	7.48
Exercisable at the end of the period (in dollars per share)	10.48			10.48
Options vested and expected to vest at end of the period (in dollars per share)	$ 7.48			$ 7.48
Weighted Average Remaining Life in Years
Outstanding balance	7 years 11 months 23 days	8 years 4 days	8 years 7 months 28 days
Options exercisable at end of period	7 years 3 months
Options vested and expected to vest at end of the period	7 years 11 months 23 days
Aggregate Intrinsic Value
Outstanding balance	$ 2		$ 68	$ 2
Options vested and expected to vest at end of the period	2			2
Stock-based compensation expense	$ 3,732	$ 3,469
Assumptions used to determine grant-date fair value
Expected Volatility	66.94%	66.94%
Weighted average risk-free interest rate	2.07%	2.80%
Expected dividend yield (as a percentage)	0.00%	0.00%
Expected term (in years)	6 years 9 months 7 days	6 years 11 months 12 days
Unrecognized costs	$ 3,059			$ 3,059
Research and development
Aggregate Intrinsic Value
Stock-based compensation expense	714	$ 625
General and administrative
Aggregate Intrinsic Value
Stock-based compensation expense	$ 3,018	$ 2,844
Weighted average
Assumptions used to determine grant-date fair value
Unrecognized costs weighted-average period	2 years 2 months 9 days
Weighted average | Options to purchase common stock
Assumptions used to determine grant-date fair value
Expected dividend yield (as a percentage)				0.00%
Minimum
Assumptions used to determine grant-date fair value
Vesting period	3 years
Maximum
Assumptions used to determine grant-date fair value
Vesting period	4 years