Convertible Preferred Stock - Schedule of Convertible Preferred Stock (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Convertible preferred stock, shares authorized	80,104,464	80,104,464
Convertible preferred stock, shares issued	78,627,369	78,627,369
Convertible preferred stock, shares outstanding	78,267,369	78,267,369
Convertible preferred stock, liquidation preference	$ 501,410	$ 501,410
Convertible preferred stock, net carrying value	$ 499,974	$ 499,974
Series A
Class of Stock [Line Items]
Convertible preferred stock, shares authorized	9,935,864	9,935,864
Convertible preferred stock, shares issued	9,263,558	9,263,558
Convertible preferred stock, shares outstanding	9,263,558	9,263,558
Convertible preferred stock, liquidation preference	$ 8,598	$ 8,598
Convertible preferred stock, net carrying value	$ 8,531	$ 8,531
Series B
Class of Stock [Line Items]
Convertible preferred stock, shares authorized	10,320,952	10,320,952
Convertible preferred stock, shares issued	10,297,182	10,297,182
Convertible preferred stock, shares outstanding	10,297,182	10,297,182
Convertible preferred stock, liquidation preference	$ 32,490	$ 32,490
Convertible preferred stock, net carrying value	$ 32,428	$ 32,428
Series C
Class of Stock [Line Items]
Convertible preferred stock, shares authorized	8,873,996	8,873,996
Convertible preferred stock, shares issued	8,873,996	8,873,996
Convertible preferred stock, shares outstanding	8,873,996	8,873,996
Convertible preferred stock, liquidation preference	$ 55,999	$ 55,999
Convertible preferred stock, net carrying value	$ 55,921	$ 55,921
Series D
Class of Stock [Line Items]
Convertible preferred stock, shares authorized	11,222,041	11,222,041
Convertible preferred stock, shares issued	11,222,041	11,222,041
Convertible preferred stock, shares outstanding	11,222,041	11,222,041
Convertible preferred stock, liquidation preference	$ 83,904	$ 83,904
Convertible preferred stock, net carrying value	$ 83,559	$ 83,559
Series E
Class of Stock [Line Items]
Convertible preferred stock, shares authorized	39,751,611	39,751,611
Convertible preferred stock, shares issued	38,970,592	38,970,592
Convertible preferred stock, shares outstanding	38,970,592	38,970,592
Convertible preferred stock, liquidation preference	$ 320,419	$ 320,419
Convertible preferred stock, net carrying value	$ 319,535	$ 319,535