Common Stock - Additional Information (Details)	1 Months Ended			12 Months Ended
	Jun. 30, 2017 USD ($) $ / shares shares	May 31, 2017 USD ($) $ / shares shares	Jul. 31, 2013 USD ($) $ / shares shares	Jan. 31, 2018 USD ($) $ / shares shares	Jan. 31, 2017 USD ($) $ / shares shares	Jan. 31, 2016 USD ($) $ / shares	Jan. 31, 2014 USD ($) Employee	Dec. 31, 2008
Class Of Stock [Line Items]
Shares reserved for future issuance				13,978,102	16,490,604
Common stock subject to repurchase				0	49,234
Common stock subject to repurchase, aggregate purchase price | $				$ 0	$ 500,000
Number of executive employees exercised stock options early | Employee							1
Number of non-executive employees exercised stock options early | Employee							1
Additional options of fully vested and exercisable options issued				2,908,420
Proceeds from exercise of stock options | $				$ 470,000	2,292,000	$ 3,132,000
Additional stock-based compensation | $				51,818,000	13,834,000	9,755,000
Aggregate intrinsic value of stock options exercised | $				$ 600,000	$ 2,900,000	$ 4,500,000
Weighted-average grant date fair value, Options granted | $ / shares				$ 7.97	$ 17.34	$ 12.36
Unrecognized stock-based compensation expense | $				$ 50,900,000
Weighted average period over which unrecognized compensation is expected to be recognized				2 years
Number of common stock purchase	1,666,665
Warrant exercisable period	10 years
Warrant exercised price | $ / shares	$ 16.44
Warrants exercisable upon additional loan drawn				34,058
Facility agreement amount | $				$ 10,000,000
Convertible Preferred Stock Warrants
Class Of Stock [Line Items]
Conversion of convertible preferred stock				68,633
Common Stock Warrants
Class Of Stock [Line Items]
Shares reserved for future issuance				1,897,562	25,000
Conversion of stock				205,897
Warrants exercisable upon additional loan drawn				34,058
Loan and Security Agreement
Class Of Stock [Line Items]
Number of common stock purchase				4,167
Fair value of warrant | $				$ 27,000
Warrant to purchase additional common stock			20,833
Exercise price of warrant | $ / shares			$ 7.80
Warrant expiration date				May 31, 2023
Issuance of additional common stock warrant | $			$ 100,000
Unamortized debt discount written off | $			$ 100,000
Full Recourse Promissory Note | Executive And Non Executive Employee
Class Of Stock [Line Items]
Debt instrument principal amount | $							$ 2,200,000
Full Recourse Promissory Note | Executive Employee
Class Of Stock [Line Items]
Additional options of fully vested and exercisable options issued		88,520
Proceeds from exercise of stock options | $	$ 1,000,000
Common stock repurchased	88,520
Common stock repurchased price per share | $ / shares	$ 13.68
Additional stock-based compensation | $				$ 500,000
Partial-recourse Promissory Note | Executive Employee
Class Of Stock [Line Items]
Debt instrument principal amount | $							$ 6,600,000
Debt instrument interest rate percentage							1.92%
Additional options of fully vested and exercisable options issued		501,104
Proceeds from exercise of stock options | $	$ 6,900,000
Common stock repurchased	501,104
Common stock repurchased price per share | $ / shares	$ 13.68
Additional stock-based compensation | $	$ 0
Minimum | Full Recourse Promissory Note | Executive And Non Executive Employee
Class Of Stock [Line Items]
Debt instrument interest rate percentage							1.62%
Maximum
Class Of Stock [Line Items]
Number of common stock purchase	1,666,665
Maximum | Full Recourse Promissory Note | Executive And Non Executive Employee
Class Of Stock [Line Items]
Debt instrument interest rate percentage							1.64%
2008 Stock Plan
Class Of Stock [Line Items]
Shares reserved for future issuance					7,688,078
2008 Stock Plan | Minimum
Class Of Stock [Line Items]
Exercise price as percentage of fair market value of common stock								100.00%
2017 Equity Incentive Plan
Class Of Stock [Line Items]
Shares reserved for future issuance		4,537,000
Exercise price as percentage of fair market value of common stock		100.00%
ISOs | 2008 Stock Plan
Class Of Stock [Line Items]
Service period								1 year
Share-based compensation arrangement by share-based payment award, vesting period								36 months
Expiration period from date of grant								10 years
ISOs | 2008 Stock Plan | Completion of One Year of Service
Class Of Stock [Line Items]
Vesting rate								25.00%
ISOs | 2017 Equity Incentive Plan
Class Of Stock [Line Items]
Service period		1 year
Share-based compensation arrangement by share-based payment award, vesting period		36 months
Expiration period from date of grant		10 years
ISOs | 2017 Equity Incentive Plan | Completion of One Year of Service | Minimum
Class Of Stock [Line Items]
Vesting rate		25.00%
Nonstatutory Stock Options | 2008 Stock Plan
Class Of Stock [Line Items]
Expiration period from date of grant								10 years
Nonstatutory Stock Options | 2017 Equity Incentive Plan
Class Of Stock [Line Items]
Expiration period from date of grant		10 years
Performance RSUs
Class Of Stock [Line Items]
Share-based compensation arrangement by share-based payment award, vesting period				2 years
2017 Employee Stock Purchase Plan
Class Of Stock [Line Items]
Shares reserved for future issuance		907,000
Exercise price as percentage of fair market value of common stock		85.00%
Percentage of outstanding common stock		1.00%
Maximum percentage of common stock eligible to purchase through payroll deductions for employees		15.00%
Maximum number of shares available for participant to purchase during period | $		$ 25,000,000
Maximum number of shares available for participant to purchase during period		2,000
Offering period		24 months
First offering period start date		Jun. 29, 2017
2017 Employee Stock Purchase Plan | IPO
Class Of Stock [Line Items]
Share price | $ / shares		$ 7.00
2017 Employee Stock Purchase Plan | Maximum
Class Of Stock [Line Items]
Number of shares increased annually to common stock shares reserve for issuance		907,000