Stockholders' Equity - Stock Options and Restricted Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Stock option and incentive plans
Total compensation cost	$ 13,588	$ 7,459
Stock option
Stock Options
Outstanding at the beginning of the period (in shares)	3,507,638	2,135,070
Granted (in shares)	1,868,066	1,905,850
Exercised (in shares)	(176,194)	(198,475)
Forfeited (in shares)	(225,726)	(180,107)
Expired (in shares)	(234,929)	(154,700)
Outstanding at the end of the period (in shares)	4,738,855	3,507,638
Exercisable at the end of the period (in shares)	2,450,061
Weighted-Average Exercise Price Per Share
Outstanding at the beginning of the period (in dollars per share)	$ 12.64	$ 15.80
Granted (in dollars per share)	13.49	9.17
Exercised (in dollars per share)	6.80	5.16
Forfeited (in dollars per share)	11.85	11.04
Expired (in dollars per share)	32.81	25.16
Outstanding at the end of the period (in dollars per share)	12.23	12.64
Exercisable at the end of the period (in dollars per share)	$ 12.56
Aggregate Intrinsic Value
Outstanding at the end of the period	$ 9,531
Exercisable at the end of the period	$ 6,521
Additional Disclosures
Weighted average remaining contractual term	8 years 2 months 12 days
Weighted average remaining contractual term exercisable	7 years 7 months 6 days
Market value per share	$ 11.88
Weighted average grant date fair value (in dollars per share)	$ 11.54	$ 7.81
Weighted-average assumptions
Expected annual dividend yield (as a percent)	0.00%	0.00%
Unrecognized compensation expense
Unrecognized compensation expense	$ 20,985
Stock option | Recognition in 2022
Unrecognized compensation expense
Unrecognized compensation expense	9,716
Stock option | Recognition in 2023
Unrecognized compensation expense
Unrecognized compensation expense	7,315
Stock option | Recognition in 2024
Unrecognized compensation expense
Unrecognized compensation expense	3,186
Stock option | Recognition in 2025
Unrecognized compensation expense
Unrecognized compensation expense	$ 768
Stock option | Minimum
Weighted-average assumptions
Expected stock price volatility (as a percent)	115.00%	116.00%
Expected term of options	5 years	5 years 3 months 18 days
Risk-free interest rate (as a percent)	0.44%	0.31%
Stock option | Maximum
Stock option and incentive plans
Vesting period	4 years
Weighted-average assumptions
Expected stock price volatility (as a percent)	124.00%	122.00%
Expected term of options	6 years 1 month 6 days	6 years 1 month 6 days
Risk-free interest rate (as a percent)	1.33%	1.73%
Restricted stock
Stock option and incentive plans
Total compensation cost	$ 279	$ 183
Unrecognized compensation expense
Unrecognized compensation cost	$ 300
Shares
Restricted stock units outstanding at the beginning of the period (in shares)	24,625	8,100
Granted (in shares)	18,400	34,000
Vested (in shares)	(17,000)	(17,475)
Restricted stock units outstanding at the end of the period (in shares	26,025	24,625
Weighted Average Grant Date Fair Value
Restricted stock units outstanding at the beginning of the period (in dollars per shares)	$ 11.41	$ 4.84
Granted (in dollars per share)	13.48	12.93
Vested (in dollars per share)	12.93	10.04
Restricted stock units outstanding at the end of the period (in dollars per shares)	$ 12.75	$ 11.41
Restricted stock | Maximum
Stock option and incentive plans
Vesting period	2 years
2005 Plan | Stock option
Stock Options
Outstanding at the end of the period (in shares)	577
2005 Plan | Stock option | Maximum
Stock option and incentive plans
Contractual life	10 years
2014 Plan | Stock option
Stock Options
Outstanding at the end of the period (in shares)	3,268,048
2014 Plan | Stock option | Maximum
Stock option and incentive plans
Contractual life	10 years
2014 Plan | Restricted stock
Shares
Restricted stock units outstanding at the beginning of the period (in shares)	26,025
Restricted stock units outstanding at the end of the period (in shares		26,025
Research and development. | Restricted stock
Stock option and incentive plans
Total compensation cost	$ 3
Research and development. | 2014 Plan
Stock option and incentive plans
Total compensation cost	4,482	$ 2,938
General and administrative | Restricted stock
Stock option and incentive plans
Total compensation cost	276	183
General and administrative | 2014 Plan
Stock option and incentive plans
Total compensation cost	$ 9,106	$ 4,521