Basic and Diluted Net Loss Per Share - Schedule of Common Equivalent Shares were Excluded From Diluted Net Loss Per Share Calculation because their Inclusion would have been Anti-Dilutive (Details) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from diluted net loss per share	6,679,809	1,968,215	10,307,069
Stock Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from diluted net loss per share	2,440,290	1,884,425	1,821,722
Shares To Be Issued Under ESSP
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from diluted net loss per share	44,320	83,790
Restricted Stock Units
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from diluted net loss per share	784,000
Convertible Senior Notes
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from diluted net loss per share	3,411,199
Series A-1 Preferred Stock Warrants
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from diluted net loss per share			130,384
Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from diluted net loss per share			8,354,963