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Financial Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($)
$ in Thousands
Jan. 31, 2019
Jan. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial assets accounted for at fair value on a recurring basis $ 336,524 $ 223,813
Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial assets accounted for at fair value on a recurring basis 336,524 223,813
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial assets accounted for at fair value on a recurring basis 279,120 157,290
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial assets accounted for at fair value on a recurring basis 57,404 66,523
Fair Value, Measurements, Recurring | Corporate Bonds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial assets accounted for at fair value on a recurring basis 261,333 151,053
Fair Value, Measurements, Recurring | Corporate Bonds | Fair Value, Inputs, Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial assets accounted for at fair value on a recurring basis 261,333 151,053
Fair Value, Measurements, Recurring | Municipal And Pre-Refunded Municipal Bonds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial assets accounted for at fair value on a recurring basis 51,232 62,303
Fair Value, Measurements, Recurring | Municipal And Pre-Refunded Municipal Bonds | Fair Value, Inputs, Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial assets accounted for at fair value on a recurring basis 51,232 62,303
Fair Value, Measurements, Recurring | Federal Government Agencies    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial assets accounted for at fair value on a recurring basis 8,062  
Fair Value, Measurements, Recurring | Federal Government Agencies | Fair Value, Inputs, Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial assets accounted for at fair value on a recurring basis 8,062  
Fair Value, Measurements, Recurring | Mutual Funds, Held in Rabbi Trust    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial assets accounted for at fair value on a recurring basis 6,751 6,237
Fair Value, Measurements, Recurring | Mutual Funds, Held in Rabbi Trust | Fair Value, Inputs, Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial assets accounted for at fair value on a recurring basis 6,751 6,237
Fair Value, Measurements, Recurring | Certificates of Deposit    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial assets accounted for at fair value on a recurring basis 3,193 4,220
Fair Value, Measurements, Recurring | Certificates of Deposit | Fair Value, Inputs, Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial assets accounted for at fair value on a recurring basis 3,193 $ 4,220
Fair Value, Measurements, Recurring | Commercial Paper    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial assets accounted for at fair value on a recurring basis 2,979  
Fair Value, Measurements, Recurring | Commercial Paper | Fair Value, Inputs, Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial assets accounted for at fair value on a recurring basis 2,979  
Fair Value, Measurements, Recurring | Treasury Bills    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial assets accounted for at fair value on a recurring basis 2,974  
Fair Value, Measurements, Recurring | Treasury Bills | Fair Value, Inputs, Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial assets accounted for at fair value on a recurring basis $ 2,974