Net Loss Per Share Attributable to Common Stockholders - Summary of Computation of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Apr. 30, 2017	Oct. 31, 2017	Oct. 31, 2016	Oct. 31, 2017	Oct. 31, 2016	Jan. 31, 2017	Jan. 31, 2016	Jan. 31, 2015
Earnings Per Share [Abstract]
Net loss		$ (37,925)	$ (23,770)	$ (120,272)	$ (80,253)	$ (105,800)	$ (101,000)	$ (69,700)
Deemed dividend to Series E and E-1 Convertible Preferred Stock	$ 6,600	0	0	(6,588)	0
Impact of adjustment to Series E, E-1 and F Convertible Preferred Stock		0	0	26,336	0
Net loss attributable to common stockholders		$ (37,925)	$ (23,770)	$ (100,524)	$ (80,253)
Weighted-average common shares outstanding		31,324,097	3,532,500	15,923,536	3,502,327
Weighted-average unvested common shares subject to repurchase		(32,584)	(72,640)	(50,465)	(89,880)
Weighted-average shares used in computing net loss per share attributable to common stockholders—basic and diluted		31,291,513	3,459,860	15,873,071	3,412,447
Net loss per share attributable to common stockholders— basic and diluted		$ (1.21)	$ (6.87)	$ (6.33)	$ (23.52)