Convertible Preferred Stock - Schedule of Convertible Preferred Stock (Details) - USD ($) $ in Thousands		Jun. 30, 2019	Dec. 31, 2018
Temporary Equity [Line Items]
Convertible preferred stock,Shares Authorized		93,762,517	93,762,517
Convertible preferred stock,Shares Issued and Outstanding		93,039,737
Convertible preferred stock,Amount		$ 561,931	$ 560,858
Convertible preferred stock,Liquidation Preference		$ 574,374	$ 572,866
Series A
Temporary Equity [Line Items]
Convertible preferred stock,Shares Authorized		4,550,000
Convertible preferred stock,Shares Issued and Outstanding		4,550,000
Convertible preferred stock,Amount		$ 12,405
Convertible preferred stock,Liquidation Preference		$ 4,550
Series B
Temporary Equity [Line Items]
Convertible preferred stock,Shares Authorized		5,645,706
Convertible preferred stock,Shares Issued and Outstanding		5,645,706
Convertible preferred stock,Amount		$ 16,018
Convertible preferred stock,Liquidation Preference		$ 9,669
Series C
Temporary Equity [Line Items]
Convertible preferred stock,Shares Authorized		4,804,227
Convertible preferred stock,Shares Issued and Outstanding		4,747,352
Convertible preferred stock,Amount		$ 14,425
Convertible preferred stock,Liquidation Preference		$ 12,521
Series D
Temporary Equity [Line Items]
Convertible preferred stock,Shares Authorized		19,269,117
Convertible preferred stock,Shares Issued and Outstanding		19,269,117
Convertible preferred stock,Amount		$ 106,905
Convertible preferred stock,Liquidation Preference		$ 106,999
Series E
Temporary Equity [Line Items]
Convertible preferred stock,Shares Authorized		15,524,350
Convertible preferred stock,Shares Issued and Outstanding		15,524,350
Convertible preferred stock,Amount		$ 93,698
Convertible preferred stock,Liquidation Preference		$ 93,750
Series E-1
Temporary Equity [Line Items]
Convertible preferred stock,Shares Authorized		17,407,441
Convertible preferred stock,Shares Issued and Outstanding		16,854,887
Convertible preferred stock,Amount	[1]	$ 73,640
Convertible preferred stock,Liquidation Preference		$ 101,785
Series F
Temporary Equity [Line Items]
Convertible preferred stock,Shares Authorized		21,761,676
Convertible preferred stock,Shares Issued and Outstanding		21,761,676
Convertible preferred stock,Amount		$ 195,013
Convertible preferred stock,Liquidation Preference		$ 195,100
Series F-1
Temporary Equity [Line Items]
Convertible preferred stock,Shares Authorized		4,800,000
Convertible preferred stock,Shares Issued and Outstanding		4,686,649
Convertible preferred stock,Amount		$ 49,827
Convertible preferred stock,Liquidation Preference		$ 50,000

[1] Includes vested Series E-1 convertible preferred stock options of $0.7 million which are not included in the shares issued and outstanding.