Share-Based Compensation - Narrative (Details) - USD ($) $ in Thousands	1 Months Ended			3 Months Ended		9 Months Ended		97 Months Ended
	Sep. 30, 2021	Feb. 28, 2021	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Oct. 31, 2019
Stock options
Stock Option and Compensation Plans
Unrecognized share-based compensation costs for option awards, net of estimated forfeitures	$ 15,300			$ 15,300		$ 15,300		$ 15,300
Expected weighted average period to recognize share-based compensation costs (in years)						3 years
Restricted stock units
Stock Option and Compensation Plans
Expected weighted average period to recognize share-based compensation costs (in years)						2 years 8 months 12 days
Unrecognized compensation costs, net of estimated forfeitures	$ 7,500			$ 7,500		$ 7,500		$ 7,500
Employee stock purchase plan
Stock Option and Compensation Plans
Common stock issued (in shares)				27,529	32,821	51,951	76,402
Proceeds from stock plans						$ 146	$ 147
2013 Plan
Stock Option and Compensation Plans
Increase in number of shares available under the Plan (in shares)						2,542,000		13,081,000
Number of shares of common stock available for issuance under the Plan (in shares)	3,031,000			3,031,000		3,031,000		3,031,000
2013 Plan | Employee stock purchase plan
Stock Option and Compensation Plans
Number of shares available for future purchase (in shares)	753,410			753,410		753,410		753,410
2019 Inducement Plan | Stock options
Stock Option and Compensation Plans
Increase in number of shares available under the Plan (in shares)	1,000,000	600,000	1,000,000
Number of shares of common stock available for issuance under the Plan (in shares)	1,314,000			1,314,000		1,314,000		1,314,000
Number of initial shares reserved for issuance under the Plan (in shares)									1,000,000