Net Loss per Share Attributable to Common Stockholders - Calculation of basic and diluted net loss per share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Jan. 31, 2020	Jan. 31, 2019	Jan. 31, 2018
Numerator:
Net loss	$ (208,359)	$ (426,458)	$ (52,276)
Less: preferred stock accretion	0	(353)	(1,461)
Net loss attributable to common stockholders	$ (208,359)	$ (426,811)	$ (53,737)
Denominator:
Weighted-average common shares outstanding (in shares)	176,704	135,163	32,294
Net loss per share attributable to common stockholders:
Basic and diluted (in usd per share)	$ (1.18)	$ (3.16)	$ (1.66)