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Financial Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($)
$ in Thousands
Oct. 31, 2017
Jan. 31, 2017
Oct. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Financial assets accounted for at fair value on a recurring basis $ 134,482 $ 155,355 $ 30,249
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 134,482 155,355 30,249
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 102,918 83,071 22,461
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 31,564 72,284 7,788
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 97,172 78,398 18,014
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 97,172 78,398 18,014
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 26,026 71,261 7,160
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 26,026 71,261 7,160
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 5,746 4,673 4,447
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 5,746 4,673 4,447
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 5,538 1,023 628
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 $ 5,538 $ 1,023 $ 628