CONSOLIDATED STATEMENT OF REDEEMABLE CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) $ in Thousands	Total	Redeemable Convertible Preferred Stock	Class A and Class B Common Stock	Additional Paid-In Capital	Treasury Stock	Accumulated Other Comprehensive Loss	Accumulated Deficit
Beginning Balance, Redeemable Convertible Preferred Stock (in shares) at Jan. 31, 2017		41,148,282
Beginning Balance, Redeemable Convertible Preferred Stock at Jan. 31, 2017		$ 345,257
Increase (Decrease) in Temporary Equity [Roll Forward]
Exercise of preferred stock warrants (in shares)		85,170
Exercise of preferred stock warrants		$ 1,171
Conversion of redeemable convertible preferred stock to common stock (in shares)		(41,233,452)	(26,953,404)
Conversion of redeemable convertible preferred stock to common stock	$ (346,428)	$ (346,428)	$ (27)	$ (346,401)
Ending Balance, Redeemable Convertible Preferred Stock (in shares) at Jan. 31, 2018		0
Ending Balance, Redeemable Convertible Preferred Stock at Jan. 31, 2018		$ 0
Common stock, beginning balance (in shares) at Jan. 31, 2017			13,093,621
Beginning balance at Jan. 31, 2017	(244,736)		$ 13	62,557	$ (1,319)	$ (364)	$ (305,623)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of common stock warrants (in shares)			99,534
Exercise of common stock warrants	1		$ 1
Stock option exercises (in shares)			1,263,722
Stock option exercises	5,597		$ 1	5,596
Repurchase of early exercised options (in shares)			(34,710)
Issuance of common stock upon initial public offering, net of offering costs (in shares)			9,200,000
Issuance of common stock upon initial public offering, net of offering costs	201,620		$ 9	201,611
Vesting of early exercised stock options	1,280			1,280
Stock-based compensation	21,235			21,235
Unrealized gain (loss) on available-for-sale securities	(88)					(88)
Foreign currency translation adjustment	293					293
Net loss	(83,973)						(83,973)
Common stock, ending balance (in shares) at Jan. 31, 2018			50,575,571
Ending balance at Jan. 31, 2018	$ 247,657		$ 51	638,680	(1,319)	(159)	(389,596)
Ending Balance, Redeemable Convertible Preferred Stock (in shares) at Jan. 31, 2019		0
Ending Balance, Redeemable Convertible Preferred Stock at Jan. 31, 2019		$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock option exercises (in shares)	3,144,202		3,144,202
Stock option exercises	$ 22,200		$ 3	22,197
Repurchase of early exercised options (in shares)			(35,668)
Vesting of early exercised stock options	1,204			1,204
Stock-based compensation	37,403			37,403
Unrealized gain (loss) on available-for-sale securities	94					94
Foreign currency translation adjustment	(109)					(109)
Vesting of restricted stock units (in shares)			263,129
Issuance of common stock under the Employee Stock Purchase Plan (in shares)			374,576
Issuance of common stock under the Employee Stock Purchase Plan	10,531			10,531
Equity component of the 0.75% convertible senior notes due 2024	81,683			81,683
Purchase of capped calls	(37,086)			(37,086)
Net loss	(99,011)						(99,011)
Common stock, ending balance (in shares) at Jan. 31, 2019			54,321,810
Ending balance at Jan. 31, 2019	$ 264,566		$ 54	754,612	(1,319)	(174)	(488,607)
Ending Balance, Redeemable Convertible Preferred Stock (in shares) at Jan. 31, 2020		0
Ending Balance, Redeemable Convertible Preferred Stock at Jan. 31, 2020		$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock option exercises (in shares)	2,163,361		2,163,361
Stock option exercises	$ 16,777		$ 3	16,774
Repurchase of early exercised options (in shares)			(5,677)
Vesting of early exercised stock options	296			296
Stock-based compensation	75,776			75,776
Unrealized gain (loss) on available-for-sale securities	91					91
Foreign currency translation adjustment	308					308
Vesting of restricted stock units (in shares)			748,061
Issuance of common stock under the Employee Stock Purchase Plan (in shares)			154,988
Issuance of common stock under the Employee Stock Purchase Plan	13,420			13,420
Equity component of the 0.75% convertible senior notes due 2024	288,998			288,998
Purchase of capped calls	(93,820)			(93,820)
Impact from 2024 Notes Partial Repurchase	(303,929)			(303,929)
Net loss	(175,522)						(175,522)
Common stock, ending balance (in shares) at Jan. 31, 2020			57,382,543
Ending balance at Jan. 31, 2020	82,858		$ 57	$ 752,127	$ (1,319)	$ 225	(668,232)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of accounting change	$ (4,103)						$ (4,103)