Annual report pursuant to Section 13 and 15(d)

Stock and Employee Benefit Plans - Narrative (Details)

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Stock and Employee Benefit Plans - Narrative (Details)
 1 Months Ended 12 Months Ended
Dec. 31, 2018
USD ($)
shares
Dec. 31, 2018
USD ($)
purchase_period
$ / shares
shares
Dec. 31, 2017
USD ($)
$ / shares
shares
Defined Benefit Plan Disclosure [Line Items]      
Stock-based compensation expense   $ 4,943,000 $ 3,046,000
Unrecognized compensation costs related to nonvested awards $ 10,800,000 $ 10,800,000  
Weighted-average compensation cost recognition period   2 years 9 months 10 days  
Research and development      
Defined Benefit Plan Disclosure [Line Items]      
Stock-based compensation expense   $ 1,984,000 844,000
General and administrative      
Defined Benefit Plan Disclosure [Line Items]      
Stock-based compensation expense   $ 2,959,000 $ 2,202,000
2016 Stock Plan      
Defined Benefit Plan Disclosure [Line Items]      
Stock available for future grant (shares) | shares 86,000 86,000  
Common stock authorized for issuance (shares) | shares 6,975,000 6,975,000  
Maximum term of stock options   10 years  
Number of options granted (shares) | shares   1,881,660 1,873,047
Weighted-average fair value (USD per share) | $ / shares   $ 5.04 $ 2.47
2018 Stock Plan      
Defined Benefit Plan Disclosure [Line Items]      
Stock available for future grant (shares) | shares 2,830,034 2,830,034  
Stock issuable under plan (shares) | shares 9,975,000 9,975,000  
Common stock authorized for issuance (shares) | shares 3,000,000 3,000,000  
Executive Officers and Senior Leadership      
Defined Benefit Plan Disclosure [Line Items]      
Stock-based compensation expense   $ 700,000 $ 800,000
Unrecognized compensation costs related to nonvested awards $ 1,700,000 1,700,000  
Executive Officers and Senior Leadership | Research and development      
Defined Benefit Plan Disclosure [Line Items]      
Stock-based compensation expense   400,000 300,000
Executive Officers and Senior Leadership | General and administrative      
Defined Benefit Plan Disclosure [Line Items]      
Stock-based compensation expense   $ 300,000 $ 500,000
Employee Stock      
Defined Benefit Plan Disclosure [Line Items]      
Common stock authorized for issuance (shares) | shares 500,000 500,000  
Discount from market price (as a percent)   85.00%  
Number of purchase periods | purchase_period   2  
Term of purchase period   6 months  
Purchases during period $ 0