Condensed Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Deficit - USD ($) shares in Thousands, $ in Thousands	Redeemable Convertible Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Accumulated Other Comprehensive Income (Loss)	Total
Beginning Balance at Jan. 31, 2018	$ 351,016
Beginning Balance (in shares) at Jan. 31, 2018	118,693
Redeemable Convertible Preferred Stock
Issuance of Series E redeemable convertible preferred stock, net of issuance costs	$ 199,896
Issuance of Series E redeemable convertible preferred stock, net of issuance costs (Shares)	12,150
Ending Balance at Jul. 31, 2018	$ 550,912
Ending Balance (in shares) at Jul. 31, 2018	130,843
Beginning Balance at Jan. 31, 2018		$ 22	$ 8,482	$ (378,948)	$ 970	$ (369,474)
Beginning Balance (in shares) at Jan. 31, 2018		44,231
Stockholders' Deficit
Issuance of common stock upon exercise of options			1,514			1,514
Issuance of common stock upon exercise of options (in shares)		1,198
Issuance of common stock related to early exercise options (in shares)		38
Issuance of restricted stock awards (in shares)		142
Vesting of early exercised options			104			104
Stock-based compensation expense			3,943			3,943
Repurchase of stock options			(2,330)			(2,330)
Net loss				(66,494)		(66,494)
Other comprehensive loss					(692)	(692)
Ending Balance at Jul. 31, 2018		$ 22	11,814	(445,543)	278	(433,429)
Ending Balance (in shares) at Jul. 31, 2018		45,609
Beginning Balance at Apr. 30, 2018	$ 351,016
Beginning Balance (in shares) at Apr. 30, 2018	118,693
Redeemable Convertible Preferred Stock
Issuance of Series E redeemable convertible preferred stock, net of issuance costs	$ 199,896
Issuance of Series E redeemable convertible preferred stock, net of issuance costs (Shares)	12,150
Ending Balance at Jul. 31, 2018	$ 550,912
Ending Balance (in shares) at Jul. 31, 2018	130,843
Beginning Balance at Apr. 30, 2018		$ 22	11,067	(412,666)	638	(400,939)
Beginning Balance (in shares) at Apr. 30, 2018		44,774
Stockholders' Deficit
Issuance of common stock upon exercise of options			763			763
Issuance of common stock upon exercise of options (in shares)		699
Issuance of restricted stock awards (in shares)		136
Vesting of early exercised options			90			90
Stock-based compensation expense			2,224			2,224
Repurchase of stock options			(2,330)			(2,330)
Net loss				(32,877)		(32,877)
Other comprehensive loss					(360)	(360)
Ending Balance at Jul. 31, 2018		$ 22	11,814	(445,543)	278	(433,429)
Ending Balance (in shares) at Jul. 31, 2018		45,609
Stockholders' Deficit
Cumulative effect of accounting change			101	(101)
Beginning Balance at Jan. 31, 2019	$ 557,912					$ 557,912
Beginning Balance (in shares) at Jan. 31, 2019	131,268					131,268
Ending Balance at Apr. 30, 2019	$ 557,912
Ending Balance (in shares) at Apr. 30, 2019	131,268
Beginning Balance at Jan. 31, 2019		$ 24	31,211	(519,126)	98	$ (487,793)
Beginning Balance (in shares) at Jan. 31, 2019		47,421
Ending Balance at Apr. 30, 2019		$ 24	36,670	(521,685)	(186)	(485,177)
Ending Balance (in shares) at Apr. 30, 2019		48,127
Beginning Balance at Jan. 31, 2019	$ 557,912					$ 557,912
Beginning Balance (in shares) at Jan. 31, 2019	131,268					131,268
Ending Balance (in shares) at Jul. 31, 2019						0
Beginning Balance at Jan. 31, 2019		$ 24	31,211	(519,126)	98	$ (487,793)
Beginning Balance (in shares) at Jan. 31, 2019		47,421
Stockholders' Deficit
Issuance of common stock upon initial public offering, net of underwriting discounts and issuance costs		$ 11	659,207			659,218
Issuance of common stock upon initial public offering, net of underwriting discounts and issuance costs ( in shares)		20,700
Conversion of redeemable convertible preferred stock to common stock upon initial public offering	$ (557,912)	$ 66	557,846			557,912
Conversion of redeemable convertible preferred stock to common stock upon initial public offering (in shares)	(131,268)	131,268
Reclassification of liability for redeemable convertible preferred stock warrants to additional paid-in capital upon initial public offering			10,559			10,559
Net exercise of common stock warrants ( in shares)		322
Issuance of common stock upon exercise of options		$ 2	8,565			8,567
Issuance of common stock upon exercise of options (in shares)		4,445
Issuance of common stock related to early exercise options (in shares)		1,037
Vesting of early exercised options			854			854
Stock-based compensation expense			33,611			33,611
Capitalized stock-based compensation			245			245
Net loss				(77,866)		(77,866)
Other comprehensive loss					(628)	(628)
Ending Balance at Jul. 31, 2019		$ 103	1,302,098	(573,574)	(530)	$ 728,097
Ending Balance (in shares) at Jul. 31, 2019		205,193
Beginning Balance at Apr. 30, 2019	$ 557,912
Beginning Balance (in shares) at Apr. 30, 2019	131,268
Ending Balance (in shares) at Jul. 31, 2019						0
Beginning Balance at Apr. 30, 2019		$ 24	36,670	(521,685)	(186)	$ (485,177)
Beginning Balance (in shares) at Apr. 30, 2019		48,127
Stockholders' Deficit
Issuance of common stock upon initial public offering, net of underwriting discounts and issuance costs		$ 11	659,207			659,218
Issuance of common stock upon initial public offering, net of underwriting discounts and issuance costs ( in shares)		20,700
Conversion of redeemable convertible preferred stock to common stock upon initial public offering	$ (557,912)	$ 66	557,846			557,912
Conversion of redeemable convertible preferred stock to common stock upon initial public offering (in shares)	(131,268)	131,268
Reclassification of liability for redeemable convertible preferred stock warrants to additional paid-in capital upon initial public offering			10,559			10,559
Net exercise of common stock warrants ( in shares)		322
Issuance of common stock upon exercise of options		$ 2	7,055			7,057
Issuance of common stock upon exercise of options (in shares)		3,739
Issuance of common stock related to early exercise options (in shares)		1,037
Vesting of early exercised options			710			710
Stock-based compensation expense			29,859			29,859
Capitalized stock-based compensation			192			192
Net loss				(51,889)		(51,889)
Other comprehensive loss					(344)	(344)
Ending Balance at Jul. 31, 2019		$ 103	$ 1,302,098	(573,574)	$ (530)	728,097
Ending Balance (in shares) at Jul. 31, 2019		205,193
Stockholders' Deficit
Cumulative effect of accounting change				$ 23,418		$ 23,418