Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Shares
Conversion of Series A convertible preferred stock (in shares)	20,697
Series A convertible preferred stock, shares outstanding	30,000				30,000
Number of common shares issued for each convertible Series A preferred stock	7,442,400				7,442,400
Deemed Dividends
Deemed dividends on Series A convertible preferred stock		$ 8,900
Series A Convertible Preferred Stock
Shares
Series A convertible preferred stock, shares outstanding	9,303				9,303
Common Stock
Stock Options
Exercised (in shares)	189,819	239,913		55,812
Shares
Conversion of convertible preferred stock to common stock (in shares)	16,557,600
Stock options
Stock option and incentive plans
Total compensation cost	$ 1,792		$ 1,253		$ 3,667	$ 3,079
Stock Options
Outstanding (in shares)	12,899,540				12,899,540
Outstanding, weighted-average exercise price (in dollars per share)	$ 3.21				$ 3.21
Granted (in shares)					5,552,700
Granted (in dollars per share)					$ 2.06
Stock options | Research and development
Stock option and incentive plans
Total compensation cost	$ 678		611		$ 1,407	1,260
Stock options | General and administrative
Stock option and incentive plans
Total compensation cost	1,114		642		2,260	1,819
Restricted stock
Stock option and incentive plans
Total compensation cost	$ 4		$ 10		$ 5	20
Shares
Issued (in shares)	21,000		0
Outstanding (in shares)	21,000				21,000
Vested (in shares)					32,400
Restricted stock | Minimum
Stock option and incentive plans
Vesting period					1 year
Restricted stock | Maximum
Stock option and incentive plans
Vesting period					3 years
Restricted stock | Research and development
Stock option and incentive plans
Total compensation cost			$ 5			10
Restricted stock | General and administrative
Stock option and incentive plans
Total compensation cost	$ 4		$ 5		$ 5	$ 10
2005 Plan
Stock option and incentive plans
Common stock reserved for issuance (in shares)	0				0
2005 Plan | Stock options
Stock Options
Outstanding (in shares)	172,276				172,276
2014 Plan
Stock option and incentive plans
Common stock reserved for issuance (in shares)	927,314				927,314
2014 Plan | Stock options
Stock Options
Outstanding (in shares)	11,037,264				11,037,264
Granted (in shares)					4,918,700
Equity Incentive Plan, Employee Inducments | Stock options
Stock Options
Granted (in shares)					634,000