Net Loss per Share Attributable to Common Stock - Summary of Anti-dilutive Stock Equivalents (Details) - shares	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share (in shares)	12,592,084	13,130,272
Stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share (in shares)	8,713,162	11,821,024
Restricted stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share (in shares)	3,588,275	1,152,718
Employee stock purchase plan
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share (in shares)	290,647	156,530