Net Loss per Share Attributable to Common Stockholders - Schedule of Potential Shares of Common Stock Excluded from Computation of Basic and Diluted Net Loss per Share Attributable to Common Stockholders (Details) - shares shares in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential shares of common stock excluded from computation of diluted net loss per share attributable to common stockholders (in shares)	34,722	198,229
Redeemable convertible preferred stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential shares of common stock excluded from computation of diluted net loss per share attributable to common stockholders (in shares)	0	165,658
Redeemable convertible preferred stock warrants
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential shares of common stock excluded from computation of diluted net loss per share attributable to common stockholders (in shares)	0	177
Shares subject to repurchase
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential shares of common stock excluded from computation of diluted net loss per share attributable to common stockholders (in shares)	6,564	7,057
Unexercised stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential shares of common stock excluded from computation of diluted net loss per share attributable to common stockholders (in shares)	21,878	25,337
Unvested restricted stock and RSUs
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential shares of common stock excluded from computation of diluted net loss per share attributable to common stockholders (in shares)	5,868	0
Shares issuable pursuant to the ESPP
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential shares of common stock excluded from computation of diluted net loss per share attributable to common stockholders (in shares)	412	0