Stock-based Compensation - Summary of Stock Option Activity (Details)
$ / shares in Units, $ in Thousands
 12 Months Ended
Dec. 31, 2020
USD ($)
$ / shares
shares
Number of Shares  
Outstanding number of stock options beginning balance (in shares) | shares 7,296,746
Number of stock options granted (in shares) | shares 89,100
Number of stock options exercised (in shares) | shares 0
Number of stock options forfeited (in shares) | shares (161,651)
Number of stock options expired (in shares) | shares (1,261,889)
Outstanding number of stock options ending balance (in shares) | shares 5,962,306
Number of stock options expected to vest (in shares) | shares 5,954,759
Number of stock options vested and exercisable (in shares) | shares 5,396,910
Weighted Average Exercise Price  
Outstanding, weighted average exercise price, beginning balance (in dollars per share) | $ / shares $ 2.48
Weighted average exercise price, granted (in dollars per share) | $ / shares 1.49
Weighted average exercise price, exercised (in dollars per share) | $ / shares 0
Weighted average exercise price, forfeited (in dollars per share) | $ / shares 2.28
Weighted average exercise price, expired in dollars per share) | $ / shares 2.73
Outstanding, weighted average exercise price, ending balance (in dollars per share) | $ / shares 2.42
Expected to vest, weighted average exercise price (in dollars per share) | $ / shares 2.42
Vested and exercisable, weighted average exercise price (in dollars per share) | $ / shares $ 2.47
Aggregate intrinsic value, outstanding | $ $ 8
Aggregate intrinsic value, expected to vest | $ 7
Aggregate intrinsic value, vested and exercisable | $ $ 0
Weighted average remaining contractual term, outstanding 4 years 8 months 19 days
Weighted average remaining contractual term, expected to vest 4 years 8 months 15 days
Weighted average remaining contractual term, vested and exercisable 4 years 4 months 20 days