Business Overview - Additional Information (Details) - USD ($)	1 Months Ended			12 Months Ended
	Aug. 31, 2017	Jul. 31, 2017	Jun. 30, 2017	Jan. 31, 2018	Jan. 31, 2017	Jan. 31, 2016	Mar. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Place of incorporation				Delaware
Period of incorporation				2008
Proceeds from initial public offering, net of underwriting discounts and commissions				$ 62,314,000	$ 0	$ 0
Conversion of convertible preferred stock			7,382,007
Net loss				157,659,000	$ 105,801,000	100,968,000
Gross proceeds related to sale of convertible preferred stock						$ 124,600,000
Line of credit facility, outstanding borrowings				69,000,000
Amount drawn down from Note Purchase Agreement				$ 68,962,000
Note Purchase Agreement | Convertible Promissory Notes | Subsequent Event
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Amount drawn down from Note Purchase Agreement							$ 25,000,000
Common Stock
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of shares issued				9,572,000
Conversion of convertible preferred stock				17,992,973
Number of shares issued				9,572,000
Common Stock | IPO
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of shares issued		8,572,000
Shares price		$ 7.00		$ 7.00
Proceeds from initial public offering, net of underwriting discounts and commissions		$ 55,800,000
Underwriting discounts and commissions		$ 4,200,000
Conversion of convertible preferred stock		17,992,973		17,992,973
Number of shares issued		8,572,000
Common Stock | IPO | Note Purchase Agreement | Convertible Promissory Notes
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Shares price			$ 7.00
Common Stock | Over-allotment Option
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of shares issued	1,000,000
Shares price	$ 7.00
Proceeds from initial public offering, net of underwriting discounts and commissions	$ 6,500,000
Underwriting discounts and commissions	$ 500,000
Number of shares issued	1,000,000