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Equity Incentive Plans - Narrative (Details) - USD ($)
$ / shares in Units, $ in Thousands
 1 Months Ended 6 Months Ended 12 Months Ended
Jun. 18, 2014
Jan. 31, 2022
Jan. 31, 2021
Sep. 30, 2020
Jul. 31, 2018
Jun. 30, 2014
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Nov. 30, 2016
Equity Incentive Plans                      
Common stock closing price (in dollars per share)             $ 1.10        
Intrinsic value of options exercised             $ 1,700 $ 2,700 $ 400    
Weighted-average grant-date estimated fair value of options granted (in dollars per share)             $ 3.92 $ 4.82 $ 1.79    
Stock-based compensation             $ 12,039 $ 10,583 $ 9,936    
Issuance of common stock for services             $ 190 $ 310 $ 312    
2014 Plan                      
Equity Incentive Plans                      
Number of shares initially authorized 1,400,000                    
Common stock reserved for future issuance (in shares) 1,500,000                    
Number of shares added to initial reserve 35,000                    
Potential increase to shares reserved for inclusion of forfeited or unexercised options from prior plan 1,200,000                    
Potential increase in shares reserved as a percentage of shares outstanding 4.00%                    
Maximum shares to be issued upon exercise of incentive stock options 10,700,000                    
Inducement Plan                      
Equity Incentive Plans                      
Common stock reserved for future issuance (in shares)                     1,000,000
Number of shares added to initial reserve     500,000                
Share of common stock subject to issued inducement grants             400,000        
Inducement Plan | Subsequent event                      
Equity Incentive Plans                      
Number of shares added to initial reserve   2,000,000                  
ESPP                      
Equity Incentive Plans                      
Common stock reserved for future issuance (in shares) 200,000                    
Potential increase in shares reserved as a percentage of shares outstanding 1.00%                    
Maximum shares to be issued under ESPP 2,200,000                    
Stock options | 2008 Plan                      
Equity Incentive Plans                      
Vesting period (in years)           4 years          
PRSUs                      
Equity Incentive Plans                      
Number of shares granted         900,000     30,000      
Number of shares vested       900,000         0 0  
Stock-based compensation             $ 0 $ 1,200      
RSUs                      
Equity Incentive Plans                      
Number of shares granted             4,144,051        
Number of shares vested             193,147        
Estimated fair value of RSUs vested             $ 800 $ 0 $ 200    
Board of directors                      
Equity Incentive Plans                      
Issuance of common stock for services (in shares)             26,000 42,000 113,000    
Issuance of common stock for services             $ 200 $ 300 $ 300