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Net Loss Per Share - Antidilutive Securities Excluded from Computation of Earnings Per Share (Details) - shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Shares subject to outstanding common stock options      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Antidilutive securities excluded from computation of earnings per share (in shares) 8,145,777 7,532,455 6,969,133
Shares subject to unvested restricted stock awards      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Antidilutive securities excluded from computation of earnings per share (in shares) 163,332 353,335 600,025
Shares issuable pursuant to the ESPP      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Antidilutive securities excluded from computation of earnings per share (in shares) 85,509 0 0