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Common Stock (Details)
 12 Months Ended
Dec. 31, 2017
Vote
$ / shares
shares
Dec. 31, 2016
$ / shares
shares
Common Stock    
Common stock, authorized (in shares) 150,000,000 150,000,000
Par value of common stock (in dollars per share) | $ / shares $ 0.001 $ 0.001
Common stock, issued (in shares) 23,645,247 18,817,572
Common Stock, outstanding (in shares) 23,645,247 18,817,572
Number of votes entitled to each common stock share | Vote 1  
Common stock dividends, declared or paid (in dollars per share) | $ / shares $ 0  
Shares reserved for future issuance (in shares) 4,218,016 3,300,018
Common stock warrants    
Common Stock    
Shares reserved for future issuance (in shares) 24,566 24,566
Stock options    
Common Stock    
Shares reserved for future issuance (in shares) 3,669,791 2,939,968
Employee Stock Purchase Plan    
Common Stock    
Shares reserved for future issuance (in shares) 523,659 335,484