Stockholders' Equity - Stock Options and Restricted Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Stock options
Stock option and incentive plans
Total compensation cost	$ 5,633	$ 4,707
Contractual life	7 years 1 month 6 days
Stock Options
Outstanding at the beginning of the period (in shares)	4,951,409	3,754,320
Granted (in shares)	4,703,000	1,372,000
Exercised (in shares)	(80,020)	(12,308)
Forfeited (in shares)	(916,205)	(162,603)
Expired (in shares)	(117,903)
Outstanding at the end of the period (in shares)	8,540,281	4,951,409
Exercisable at the end of the period (in shares)	4,431,560
Exercisable and expected to vest at the end of the year (in shares)	8,540,281
Weighted-Average Exercise Price Per Share
Outstanding at the beginning of the period (in dollars per share)	$ 5.62	$ 5.22
Granted (in dollars per share)	2.31	6.60
Exercised (in dollars per share)	1.22	1.18
Forfeited (in dollars per share)	4.21	5.00
Expired (in dollars per share)	8.23
Outstanding at the end of the period (in dollars per share)	3.95	5.62
Exercisable at the end of the period (in dollars per share)	5.04
Exercisable and expected to vest at the end of the year (in dollars per share)	$ 3.95
Aggregate Intrinsic Value
Outstanding at the end of the period	$ 3,774
Exercisable at the end of the period	1,447
Exercisable and expected to vest at the end of the year	$ 3,774
Additional Disclosures
Weighted average remaining contractual term	8 years 1 month 6 days
Weighted average remaining contractual term exercisable	7 years 1 month 6 days
Market value per share		2.16
Weighted average grant date fair value (in dollars per share)	$ 1.90	$ 5.51
Weighted-average assumptions
Expected annual dividend yield (as a percent)	0.00%	0.00%
Unrecognized compensation expense
Unrecognized compensation expense	$ 8,291
Stock options | Recognition in 2020
Unrecognized compensation expense
Unrecognized compensation expense	3,981
Stock options | Recognition in 2021
Unrecognized compensation expense
Unrecognized compensation expense	2,941
Stock options | Recognition in 2022
Unrecognized compensation expense
Unrecognized compensation expense	1,065
Stock options | Recognition in 2023
Unrecognized compensation expense
Unrecognized compensation expense	$ 304
Stock options | Minimum
Weighted-average assumptions
Expected stock price volatility (as a percent)	104.70%	106.64%
Expected term of options	5 years 1 month 28 days	5 years 2 months 12 days
Risk-free interest rate (as a percent)	1.51%	2.32%
Stock options | Maximum
Weighted-average assumptions
Expected stock price volatility (as a percent)	118.87%	113.19%
Expected term of options	6 years 1 month 6 days	6 years 1 month 6 days
Risk-free interest rate (as a percent)	2.59%	3.10%
Restricted stock
Stock option and incentive plans
Total compensation cost	$ 39	$ 81
Vesting period	1 year
Unrecognized compensation expense
Unrecognized compensation cost	$ 0
Shares
Restricted stock units outstanding at the beginning of the period (in shares)	105,200	239,800
Granted (in shares)		0
Vested (in shares)	(52,600)	(120,600)
Forfeited (in shares)	(20,200)	(14,000)
Restricted stock units outstanding at the end of the period (in shares	32,400	105,200
Weighted Average Grant Date Fair Value
Restricted stock units outstanding at the beginning of the period (in dollars per shares)	$ 1.21	$ 1.21
Vested (in dollars per share)	1.21	1.21
Forfeited (in dollars per share)	1.21	1.21
Restricted stock units outstanding at the end of the period (in dollars per shares)	$ 1.21	$ 1.21
2005 Plan | Stock options
Stock Options
Outstanding at the end of the period (in shares)	330,450
2005 Plan | Stock options | Maximum
Stock option and incentive plans
Contractual life	10 years
Vesting period	4 years
Additional Disclosures
Weighted average remaining contractual term exercisable	10 years
2014 Plan | Stock options
Stock Options
Outstanding at the end of the period (in shares)	7,121,331
2014 Plan | Stock options | Maximum
Stock option and incentive plans
Contractual life	10 years
Vesting period	4 years
Additional Disclosures
Weighted average remaining contractual term exercisable	10 years
2014 Plan | Restricted stock
Stock Options
Outstanding at the end of the period (in shares)	32,400
Research and development | Stock options
Stock option and incentive plans
Total compensation cost	$ 2,563	$ 1,712
Research and development | Restricted stock
Stock option and incentive plans
Total compensation cost	19	19
General and administrative | Stock options
Stock option and incentive plans
Total compensation cost	3,070	2,995
General and administrative | Restricted stock
Stock option and incentive plans
Total compensation cost	$ 20	$ 62