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Gross Unrealized Losses and Fair Value of Marketable Securities (Detail) - USD ($)
$ in Thousands
Jan. 31, 2017
Jan. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Fair Value $ 80,641 $ 36,105
Less Than 12 Months, Unrealized Losses (145) (303)
12 Months or Greater, Fair Value   1,532
12 Months or Greater, Unrealized Losses   (6)
Total, Fair Value 80,641 37,637
Total, Unrealized Losses (145) (309)
Corporate Bonds    
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Fair Value 61,612 30,745
Less Than 12 Months, Unrealized Losses (123) (54)
12 Months or Greater, Fair Value   1,098
12 Months or Greater, Unrealized Losses   (6)
Total, Fair Value 61,612 31,843
Total, Unrealized Losses (123) (60)
Municipal And Pre-Refunded Municipal Bonds    
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Fair Value 18,713 997
Less Than 12 Months, Unrealized Losses (21) (2)
12 Months or Greater, Fair Value   434
Total, Fair Value 18,713 1,431
Total, Unrealized Losses (21) (2)
Mutual Funds, Held in Rabbi Trust    
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Fair Value 316 4,363
Less Than 12 Months, Unrealized Losses (1) (247)
Total, Fair Value 316 4,363
Total, Unrealized Losses $ (1) $ (247)