Stock-Based Compensation - Stock options (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Options
Outstanding balance at the beginning of the period (in shares)	53,228	59,274
Granted (in shares)	2,220,858
Exercised (in shares)	(3,506)
Forfeited (in shares)	(12,959)	(6,046)
Outstanding balance at the end of the period (in shares)	2,257,621	53,228	59,274
Options exercisable at the end of the period (in shares)	716,316
Options vested and expected to vest at end of period (in shares)	2,257,621
Weighted Average Exercise Price Per Share
Outstanding balance at the beginning of the period (in dollars per share)	$ 4.93	$ 4.95
Granted (in dollars per share)	12.51
Exercised (in dollars per share)	5.55
Forfeited (in dollars per share)	5.78	5.14
Outstanding balance at the end of the period (in dollars per share)	12.38	$ 4.93	$ 4.95
Exercisable at the end of the period (in dollars per share)	19.62
Options vested and expected to vest at end of the period (in dollars per share)	$ 12.38
Weighted Average Remaining Life in Years
Outstanding balance	8 years 7 months 28 days	7 years	7 years 10 months 24 days
Options exercisable at end of period	7 years 4 months 6 days
Options vested and expected to vest at end of the period	8 years 7 months 28 days
Aggregate Intrinsic Value
Outstanding balance	$ 68	$ 213
Options exercisable at the end of the period	65
Options vested and expected to vest at end of the period	68
Stock-based compensation expense	$ 5,600	45
Assumptions used to determine grant-date fair value
Vesting period subsequent to the first anniversary of the grant date	3 years
Expiration period	10 years
Compensation cost related to the non-vested awards not yet recognized	$ 6,834
Weighted average period for recognition of compensation cost	2 years 6 months 26 days
Vesting on first anniversary
Assumptions used to determine grant-date fair value
Options vesting on the first anniversary date of the grants (as a percent)	25.00%
Vesting period	1 year
Research and development
Aggregate Intrinsic Value
Stock-based compensation expense	$ 1,834	35
General and administrative
Aggregate Intrinsic Value
Stock-based compensation expense	$ 3,766	$ 10
Stock Options | Weighted average
Assumptions used to determine grant-date fair value
Expected Volatility	68.70%
Weighted average risk-free interest rate	2.10%
Expected dividend yield (as a percentage)	0.00%
Expected term (in years)	6 years 7 months 6 days