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Securities Available for Sale - Additional Information (Detail)
12 Months Ended
Jan. 31, 2019
USD ($)
Debt Securities, Available-for-sale [Line Items]  
Impairment charges on available for sale security $ 0
Minimum [Member]  
Debt Securities, Available-for-sale [Line Items]  
Original maturity of short-term investments 90 days
Anticipated maturity period 1 month
Maximum [Member]  
Debt Securities, Available-for-sale [Line Items]  
Anticipated maturity period 13 months