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, 2020	Sep. 30, 2019
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)	47,534	34,722
Convertible senior notes
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)	15,363	0
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)	4,354	6,564
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)	19,065	21,878
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)	8,524	5,868
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)	228	412