Net Loss per Share - Anti-dilutive Securities (Details) - shares	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Anti-dilutive securities
Potential common shares excluded from computation of diluted net loss per share (in shares)	18,864,914	59,048,499
Employee and Non-Employee Stock Options
Anti-dilutive securities
Potential common shares excluded from computation of diluted net loss per share (in shares)	18,452,393	13,566,354
RSUs
Anti-dilutive securities
Potential common shares excluded from computation of diluted net loss per share (in shares)	412,521
Options to Purchase Series E-1 Preferred Stock [Member]
Anti-dilutive securities
Potential common shares excluded from computation of diluted net loss per share (in shares)		837,835
Common stock warrants
Anti-dilutive securities
Potential common shares excluded from computation of diluted net loss per share (in shares)		106,250
Redeemable convertible preferred stock warrants
Anti-dilutive securities
Potential common shares excluded from computation of diluted net loss per share (in shares)		167,500
Redeemable convertible preferred stock
Anti-dilutive securities
Potential common shares excluded from computation of diluted net loss per share (in shares)		44,370,560