Net Loss per Share Attributable to Common Stockholders - Schedule of Antidilutive Securities (Details) - shares shares in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities	31,835	28,055	32,254	30,762
Convertible and non-convertible redeemable preferred stock and convertible notes
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities	27,264	24,512	27,264	27,219
Stock options to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities	4,571	3,454	4,990	3,454
Convertible redeemable common stock warrants
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities	0	89	0	89