Net Loss Per Share Attributable to Common Stockholders - Weighted-Average Anti-Dilutive Shares of Common Stock Excluded from the Calculation of Diluted Net Loss Per Share (Details) - shares shares in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Weighted-average anti-dilutive shares of common stock (in shares)	108,993	466,704
Redeemable convertible preferred stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Weighted-average anti-dilutive shares of common stock (in shares)	0	308,373
Outstanding stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Weighted-average anti-dilutive shares of common stock (in shares)	52,581	76,584
Unvested restricted stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Weighted-average anti-dilutive shares of common stock (in shares)	56,412	81,498
Redeemable convertible preferred stock warrants
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Weighted-average anti-dilutive shares of common stock (in shares)	0	249