CONSOLIDATED STATEMENTS OF REDEEMABLE CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) $ in Thousands	Total	Series F Redeemable Convertible Preferred Stock	Series B Redeemable Convertible Preferred Stock	Redeemable Convertible Preferred Stock	Common Stock Class A Common Stock	Common Stock Class B Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning balance (in shares) at Jan. 31, 2015	53,223,503
Beginning balance at Jan. 31, 2015	$ 154,530
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of redeemable convertible preferred stock (in shares)		6,241,936
Issuance of redeemable convertible preferred stock		$ 73,424
Ending balance (in shares) at Jan. 31, 2016	59,465,439
Ending balance at Jan. 31, 2016	$ 227,954
Beginning balance (in shares) at Jan. 31, 2015					0	18,042,388
Beginning balance at Jan. 31, 2015	(117,198)				$ 0	$ 2	$ 10,908	$ (10)	$ (128,098)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options (in shares)						1,291,099
Issuance of common stock upon exercise of stock options	1,194						1,194
Issuance of common stock pursuant to charitable donation (in shares)						17,433
Issuance of common stock pursuant to charitable donation	132						132
Repurchases of unvested common stock (in shares)						(25,846)
Vesting of early exercised stock options	1,014						1,014
Stock-based compensation	10,076						10,076
Excess tax benefits from share-based compensation	69						69
Other comprehensive loss	(47)							(47)
Net loss	(76,302)				$ 0	$ (76,302)			(76,302)
Ending balance (in shares) at Jan. 31, 2016					0	19,325,074
Ending balance at Jan. 31, 2016	$ (181,062)				$ 0	$ 2	23,393	(57)	(204,400)
Ending balance (in shares) at Jan. 31, 2017	59,465,439	6,241,936	11,986,055
Ending balance at Jan. 31, 2017	$ 227,954	$ 73,424	$ 16,420
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options (in shares)						980,074
Issuance of common stock upon exercise of stock options	2,001						2,001
Issuance of common stock pursuant to charitable donation (in shares)						13,935
Issuance of common stock pursuant to charitable donation	129						129
Repurchases of unvested common stock (in shares)						(25,745)
Vesting of early exercised stock options	1,297						1,297
Stock-based compensation	17,649						17,649
Other comprehensive loss	(110)							(110)
Net loss	(83,509)				$ 0	$ (83,509)			(83,509)
Ending balance (in shares) at Jan. 31, 2017					0	20,293,338
Ending balance at Jan. 31, 2017	(243,605)				$ 0	$ 2	44,469	(167)	(287,909)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative-effect adjustment in connection with the adoption of ASU 2016-09	0						200		(200)
Issuance of redeemable convertible preferred stock (in shares)			26,201
Issuance of redeemable convertible preferred stock			$ 408
Conversion of convertible preferred stock to common stock in connection with initial public offering (in shares)				(59,491,640)		59,491,640
Conversion of convertible preferred stock to common stock in connection with initial public offering	$ 228,362			$ (228,362)		$ 6	228,356
Ending balance (in shares) at Jan. 31, 2018	0
Ending balance at Jan. 31, 2018	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of stock (in shares)					12,650,000
Issuance of stock	$ 194,345				$ 1		194,344
Issuance of common stock upon exercise of stock options (in shares)	9,180,242				8,277,412	902,830
Issuance of common stock upon exercise of stock options	$ 33,363				$ 1		33,362
Issuance of common stock upon net exercise of warrant (in shares)						168,750
Issuance of common stock pursuant to charitable donation (in shares)					24,287	0
Issuance of common stock pursuant to charitable donation	708						708
Issuance of common stock under employee stock purchase plan (in shares)					569,373
Issuance of common stock under employee stock purchase plan	8,369						8,369
Repurchases of unvested common stock (in shares)					(2,813)	(2,313)
Conversion of Class B common stock to Class A common stock (in shares)					49,091,639	(49,091,639)
Conversion of Class B common stock to Class A common stock	0				$ 5	$ (5)
Vesting of early exercised stock options	1,335						1,335
Conversion of convertible preferred stock to common stock in connection with initial public offering (in shares)				(59,491,640)		59,491,640
Conversion of convertible preferred stock to common stock in connection with initial public offering	228,362			$ (228,362)		$ 6	228,356
Issuance of common stock and restricted stock in connection with acquisition (in shares)						1,598,500
Issuance of common stock and restricted stock in connection with acquisition	3,652						3,652
Stock-based compensation	50,858						50,858
Other comprehensive loss	558							558
Net loss	(114,359)				$ (33,293)	$ (81,066)			(114,359)
Ending balance (in shares) at Jan. 31, 2018					70,609,898	33,361,106
Ending balance at Jan. 31, 2018	$ 163,586				$ 7	$ 3	$ 565,653	$ 391	$ (402,468)