Net Loss per Share Attributable to Common Stockholders - Schedule of Antidilutive Securities (Details) - shares shares in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities	33,733	88,465	33,064	88,465
Convertible and non-convertible redeemable preferred stock and convertible notes
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities	27,253	85,945	27,253	85,945
Stock options to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities	6,480	2,148	5,811	2,148
Convertible redeemable preferred stock warrants
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities	0	60	0	60
Convertible redeemable common stock warrants
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities	0	312	0	312