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Summary of Significant Accounting Policies - Net Loss Per Share and Segment Information (Details)
12 Months Ended
Dec. 31, 2017
segment
shares
Dec. 31, 2016
shares
Dec. 31, 2015
shares
Antidilutive common stock equivalents excluded from computation of diluted net loss per share      
Anti-dilutive securities excluded from the calculation of diluted net loss per share (in shares) 2,870,192 2,294,735 1,782,959
Segment Information      
Number of operating segments | segment 1    
Stock options      
Antidilutive common stock equivalents excluded from computation of diluted net loss per share      
Anti-dilutive securities excluded from the calculation of diluted net loss per share (in shares) 2,845,626 2,270,169 1,723,554
Common stock warrants      
Antidilutive common stock equivalents excluded from computation of diluted net loss per share      
Anti-dilutive securities excluded from the calculation of diluted net loss per share (in shares) 24,566 24,566 59,405