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Gross Unrealized Losses and Fair Value of Marketable Securities (Detail) (USD $)
In Thousands, unless otherwise specified
Jan. 31, 2014
Jan. 31, 2013
Marketable Securities And Unrealized Loss Position [Line Items]    
Less Than 12 Months, Fair Value $ 183,433 $ 125,755
Less Than 12 Months, Unrealized Losses (250) (165)
12 Months or Greater, Fair Value   5,987
12 Months or Greater, Unrealized Losses   (595)
Total, Fair Value 183,433 131,742
Total, Unrealized Losses (250) (760)
Corporate Bonds
    
Marketable Securities And Unrealized Loss Position [Line Items]    
Less Than 12 Months, Fair Value 147,731 74,537
Less Than 12 Months, Unrealized Losses (203) (85)
Total, Fair Value 147,731 74,537
Total, Unrealized Losses (203) (85)
Municipal And Pre-Refunded Municipal Bonds
    
Marketable Securities And Unrealized Loss Position [Line Items]    
Less Than 12 Months, Fair Value 6,291 42,826
Less Than 12 Months, Unrealized Losses (10) (77)
12 Months or Greater, Fair Value   1,413
Total, Fair Value 6,291 44,239
Total, Unrealized Losses (10) (77)
US Treasury Bill Securities
    
Marketable Securities And Unrealized Loss Position [Line Items]    
Less Than 12 Months, Fair Value 6,606  
Total, Fair Value 6,606  
Certificates of Deposit
    
Marketable Securities And Unrealized Loss Position [Line Items]    
Less Than 12 Months, Fair Value 12,746 3,400
Less Than 12 Months, Unrealized Losses (3)  
12 Months or Greater, Fair Value   244
Total, Fair Value 12,746 3,644
Total, Unrealized Losses (3)  
Commercial Paper
    
Marketable Securities And Unrealized Loss Position [Line Items]    
Less Than 12 Months, Fair Value 6,640 2,994
Less Than 12 Months, Unrealized Losses (1) (1)
Total, Fair Value 6,640 2,994
Total, Unrealized Losses (1) (1)
US Government Corporations and Agencies Securities
    
Marketable Securities And Unrealized Loss Position [Line Items]    
Less Than 12 Months, Fair Value 1,753 1,998
Less Than 12 Months, Unrealized Losses   (2)
Total, Fair Value 1,753 1,998
Total, Unrealized Losses   (2)
Mutual funds, held in rabbi trust
    
Marketable Securities And Unrealized Loss Position [Line Items]    
Less Than 12 Months, Fair Value 1,666  
Less Than 12 Months, Unrealized Losses (33)  
Total, Fair Value 1,666  
Total, Unrealized Losses (33)  
Auction Rate Securities
    
Marketable Securities And Unrealized Loss Position [Line Items]    
12 Months or Greater, Fair Value   4,330
12 Months or Greater, Unrealized Losses   (595)
Total, Fair Value   4,330
Total, Unrealized Losses   $ (595)