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Stockholders' Equity - Equity Incentive Plans (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 15, 2016
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Equity Incentive Plans        
Stock-based compensation   $ 3,232 $ 4,367 $ 6,387
Research and Development Expense        
Equity Incentive Plans        
Stock-based compensation   825 1,077 1,954
General and Administrative Expense        
Equity Incentive Plans        
Stock-based compensation   $ 2,407 $ 3,290 $ 4,433
2008 Plan and 2013 Plan        
Equity Incentive Plans        
Authorized (in shares) 500,000      
2008 Plan and 2013 Plan | Maximum        
Equity Incentive Plans        
Term of award   10 years    
Vesting period   4 years