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Financial Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($)
$ in Thousands
Apr. 30, 2019
Jan. 31, 2019
Apr. 30, 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 $ 323,057 $ 336,524 $ 201,446
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 323,057 336,524 201,446
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 253,821 279,120 134,771
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 69,236 57,404 66,675
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 240,783 261,333 128,288
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 240,783 261,333 128,288
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 63,186 51,232 62,455
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 63,186 51,232 62,455
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 7,941 6,751 6,483
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 7,941 6,751 6,483
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 5,097 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 5,097 8,062  
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 3,053 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 3,053 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 2,997 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 $ 2,997 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