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Gross Unrealized Losses and Fair Value of Marketable Securities (Detail) (USD $)
In Thousands, unless otherwise specified
Jan. 31, 2013
Jan. 31, 2012
Marketable Securities And Unrealized Loss Position [Line Items]    
Less Than 12 Months, Fair Value $ 125,755 $ 58,701
Less Than 12 Months, Unrealized Losses (165) (111)
12 Months or Greater, Fair Value 5,987 23,151
12 Months or Greater, Unrealized Losses (595) (2,785)
Total, Fair Value 131,742 81,852
Total, Unrealized Losses (760) (2,896)
Corporate Bonds
    
Marketable Securities And Unrealized Loss Position [Line Items]    
Less Than 12 Months, Fair Value 74,537 45,089
Less Than 12 Months, Unrealized Losses (85) (100)
Total, Fair Value 74,537 45,089
Total, Unrealized Losses (85) (100)
Municipal And Pre-Refunded Municipal Bonds
    
Marketable Securities And Unrealized Loss Position [Line Items]    
Less Than 12 Months, Fair Value 42,826 9,985
Less Than 12 Months, Unrealized Losses (77) (9)
12 Months or Greater, Fair Value 1,413 2,954
12 Months or Greater, Unrealized Losses   (7)
Total, Fair Value 44,239 12,939
Total, Unrealized Losses (77) (16)
Certificates of Deposit
    
Marketable Securities And Unrealized Loss Position [Line Items]    
Less Than 12 Months, Fair Value 3,400 1,489
Less Than 12 Months, Unrealized Losses   (1)
12 Months or Greater, Fair Value 244  
Total, Fair Value 3,644 1,489
Total, Unrealized Losses   (1)
Commercial Paper
    
Marketable Securities And Unrealized Loss Position [Line Items]    
Less Than 12 Months, Fair Value 2,994  
Less Than 12 Months, Unrealized Losses (1)  
Total, Fair Value 2,994  
Total, Unrealized Losses (1)  
US Treasury Bill Securities
    
Marketable Securities And Unrealized Loss Position [Line Items]    
Less Than 12 Months, Fair Value   1,039
Total, Fair Value   1,039
US Government Corporations and Agencies Securities
    
Marketable Securities And Unrealized Loss Position [Line Items]    
Less Than 12 Months, Fair Value 1,998 1,099
Less Than 12 Months, Unrealized Losses (2) (1)
Total, Fair Value 1,998 1,099
Total, Unrealized Losses (2) (1)
Auction Rate Securities
    
Marketable Securities And Unrealized Loss Position [Line Items]    
12 Months or Greater, Fair Value 4,330 20,197
12 Months or Greater, Unrealized Losses (595) (2,778)
Total, Fair Value 4,330 20,197
Total, Unrealized Losses $ (595) $ (2,778)