Short-Term Investments - Additional Information (Detail) - USD ($)	9 Months Ended	12 Months Ended
Oct. 31, 2018	Jan. 31, 2018
Investments Debt And Equity Securities [Abstract]
Other-than-temporary impairment losses on investments	$ 0	$ 0

The amount by which the fair value of an investment in debt and equity securities categorized as Available-for-sale is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

No definition available.

Name:	us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsAvailableforsaleSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration