Consolidated Statements of Convertible Preferred Stock and Stockholders’ Equity (Deficit) - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning balance (in shares) at Jan. 31, 2015	122,281
Beginning balance at Jan. 31, 2015	$ 543,940
Increase (Decrease) in Temporary Equity [Roll Forward]
Conversion of convertible preferred stock to common stock upon initial public offering (in shares)	(122,281)
Conversion of convertible preferred stock to common stock upon initial public offering	$ (543,940)
Ending balance (in shares) at Jan. 31, 2016	0
Ending balance at Jan. 31, 2016	$ 0
Beginning balance (in shares) at Jan. 31, 2015		36,465
Beginning balance at Jan. 31, 2015	(299,830)	$ 4	$ 41,749	$ 0	$ (341,583)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Conversion of convertible preferred stock to common stock upon initial public offering (in shares)		122,281
Conversion of convertible preferred stock to common stock upon initial public offering	543,940	$ 12	543,928
Issuance of common stock upon initial public offering, net of offering costs of $4,539 (in shares)		28,750
Issuance of common stock upon initial public offering, net of offering costs of $4,539	455,138	$ 3	455,135
Issuance of common stock to Pure Good Foundation (in shares)		700
Issuance of common stock to Pure Good Foundation	11,900		11,900
Issuance of common stock upon exercise of stock options, net of repurchases (in shares)		2,313
Issuance of common stock upon exercise of stock options, net of repurchases	6,008	$ 0	6,008
Stock-based compensation expense	58,225		58,225
Vesting of early exercised stock options	1,725		1,725
Net loss	(213,752)				(213,752)
Ending balance (in shares) at Jan. 31, 2016		190,509
Ending balance at Jan. 31, 2016	$ 563,354	$ 19	1,118,670	0	(555,335)
Ending balance (in shares) at Jan. 31, 2017	0
Ending balance at Jan. 31, 2017	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative-effect adjustment from adoption of ASU 2016-09 | Accounting Standards Update 2016-09	0		2,079		(2,079)
Issuance of common stock upon exercise of stock options, net of repurchases (in shares)		10,180
Issuance of common stock upon exercise of stock options, net of repurchases	15,031	$ 1	15,030
Stock-based compensation expense	116,668		116,668
Vesting of early exercised stock options	3,399		3,399
Vesting of restricted stock units (in shares)		1,238
Vesting of restricted stock units	0	$ 0	0
Common stock issued under employee stock purchase plan (in shares)		2,437
Common stock issued under employee stock purchase plan	25,606		25,606
Other comprehensive income (loss)	(562)			(562)
Net loss	(245,066)				(245,066)
Ending balance (in shares) at Jan. 31, 2017		204,364
Ending balance at Jan. 31, 2017	$ 478,430	$ 20	1,281,452	(562)	(802,480)
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 common stock upon exercise of stock options, net of repurchases (in shares)		8,814
Issuance of common stock upon exercise of stock options, net of repurchases	24,581	$ 1	24,580
Stock-based compensation expense	150,673		150,673
Vesting of early exercised stock options	1,042		1,042
Vesting of restricted stock units (in shares)		5,278
Vesting of restricted stock units	0	$ 1	(1)
Common stock issued under employee stock purchase plan (in shares)		2,523
Common stock issued under employee stock purchase plan	22,137		22,137
Other comprehensive income (loss)	(1,355)			(1,355)
Net loss	(177,602)				(177,602)
Ending balance (in shares) at Jan. 31, 2018		220,979
Ending balance at Jan. 31, 2018	$ 497,906	$ 22	$ 1,479,883	$ (1,917)	$ (980,082)