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Stockholders' Equity - Narrative (Details) - USD ($)
 1 Months Ended 3 Months Ended 6 Months Ended
May 31, 2018
Jun. 30, 2019
Jun. 30, 2019
May 13, 2016
2016 Incentive Plan        
Class of Stock [Line Items]        
Number of shares authorized (shares)   16,000,000 16,000,000 16,000,000
Number of potential shares authorized for issuance under share based compensation plan (shares)       2,000,000
Number of shares available for grant (shares)   8,065,073 8,065,073  
Number of shares of unvested restricted stock units and options outstanding (shares)   1,573,526 1,573,526  
Common stock, other shares, outstanding (shares)   5,222,849 5,222,849  
Award vesting period     3 years  
Maximum contractual term     10 years  
2007 Incentive Plan        
Class of Stock [Line Items]        
Common stock, other shares, outstanding (shares)   5,547,470 5,547,470  
Award vesting period     3 years  
Maximum contractual term     10 years  
Number of shares of unvested restricted stock units (shares)     6,666  
Common stock | Sales Agreement        
Class of Stock [Line Items]        
Maximum authorized amount $ 100,000,000.0      
Agent fee 3.00%      
Aggregate number of shares issued (shares)   476,600 659,800  
Stock Sale Agreement weighted average price per share   $ 2.97 $ 3.55  
Aggregate proceeds   $ 1,400,000 $ 2,300,000  
Stock sales agreement, remaining authorized sales amount   $ 69,700,000 $ 69,700,000