Equity Incentive Plans - Stock options (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Options
Outstanding balance at the beginning of the period (in shares)	2,257,621
Forfeited (in shares)	(7,541)
Outstanding balance at the end of the period (in shares)	2,250,080		2,257,621
Options exercisable at the end of the period (in shares)	1,165,704
Options vested and expected to vest at end of period (in shares)	2,250,080
Weighted Average Exercise Price Per Share
Outstanding balance at the beginning of the period (in dollars per share)	$ 12.38
Forfeited (in dollars per share)	7.24
Outstanding balance at the end of the period (in dollars per share)	12.40		$ 12.38
Exercisable at the end of the period (in dollars per share)	15.79
Options vested and expected to vest at end of the period (in dollars per share)	$ 12.40
Weighted Average Remaining Life in Years
Outstanding balance	8 years 4 months 28 days		8 years 7 months 28 days
Options exercisable at end of period	7 years 9 months 11 days
Options vested and expected to vest at end of the period	8 years 4 months 28 days
Aggregate Intrinsic Value
Outstanding balance	$ 895		$ 68
Options exercisable at the end of the period	242
Options vested and expected to vest at end of the period	895
Stock-based compensation expense	$ 728	$ 3,623
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
Expiration period	10 years
Compensation cost related to the non-vested awards not yet recognized	$ 6,084
Weighted average period for recognition of compensation cost	2 years 4 months 2 days
Research and development
Aggregate Intrinsic Value
Stock-based compensation expense	$ 117	1,678
General and administrative
Aggregate Intrinsic Value
Stock-based compensation expense	$ 611	$ 1,945
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