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Amortized Cost, Gross Unrealized Gains (Losses) and Fair Value of Available-For-Sale Securities (Detail) (USD $)
In Thousands, unless otherwise specified
Jan. 31, 2014
Jan. 31, 2013
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 647,483 $ 378,393
Unrealized Gains 1,002 438
Unrealized (Losses) (250) (760)
Fair Value 648,235 378,071
Short-term Investments
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 281,674 228,286
Unrealized Gains 184 242
Unrealized (Losses) (45) (42)
Fair Value 281,813 228,486
Long Term Investments
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 365,809 150,107
Unrealized Gains 818 196
Unrealized (Losses) (205) (718)
Fair Value 366,422 149,585
Corporate Bonds
    
Schedule of Available-for-sale Securities [Line Items]    
Fair Value 309,423 152,775
Corporate Bonds | Short-term Investments
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 100,856 88,432
Unrealized Gains 56 106
Unrealized (Losses) (41) (23)
Fair Value 100,871 88,515
Corporate Bonds | Long Term Investments
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 208,446 64,219
Unrealized Gains 268 102
Unrealized (Losses) (162) (61)
Fair Value 208,552 64,260
Municipal And Pre-Refunded Municipal Bonds
    
Schedule of Available-for-sale Securities [Line Items]    
Fair Value 211,437 116,364
Municipal And Pre-Refunded Municipal Bonds | Short-term Investments
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 85,000 63,355
Unrealized Gains 98 85
Unrealized (Losses) (2) (17)
Fair Value 85,096 63,423
Municipal And Pre-Refunded Municipal Bonds | Long Term Investments
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 125,934 52,925
Unrealized Gains 415 76
Unrealized (Losses) (8) (60)
Fair Value 126,341 52,941
US Treasury Bill Securities
    
Schedule of Available-for-sale Securities [Line Items]    
Fair Value 46,455 41,105
US Treasury Bill Securities | Short-term Investments
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 24,873 21,354
Unrealized Gains 10 14
Fair Value 24,883 21,368
US Treasury Bill Securities | Long Term Investments
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 21,551 19,724
Unrealized Gains 21 13
Fair Value 21,572 19,737
Certificates of Deposit
    
Schedule of Available-for-sale Securities [Line Items]    
Fair Value 39,854 43,235
Certificates of Deposit | Short-term Investments
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 35,844 40,870
Unrealized Gains 13 25
Unrealized (Losses) (1)  
Fair Value 35,856 40,895
Certificates of Deposit | Long Term Investments
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 4,000 2,340
Unrealized (Losses) (2)  
Fair Value 3,998 2,340
Commercial Paper
    
Schedule of Available-for-sale Securities [Line Items]    
Fair Value 35,107 10,781
Commercial Paper | Short-term Investments
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 35,101 10,775
Unrealized Gains 7 8
Unrealized (Losses) (1) (2)
Fair Value 35,107 10,781
US Government Corporations and Agencies Securities
    
Schedule of Available-for-sale Securities [Line Items]    
Fair Value 4,293 9,481
US Government Corporations and Agencies Securities | Short-term Investments
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost   3,500
Unrealized Gains   4
Fair Value   3,504
US Government Corporations and Agencies Securities | Long Term Investments
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 4,287 5,974
Unrealized Gains 6 5
Unrealized (Losses)   (2)
Fair Value 4,293 5,977
Mutual funds, held in rabbi trust
    
Schedule of Available-for-sale Securities [Line Items]    
Fair Value 1,666  
Mutual funds, held in rabbi trust | Long Term Investments
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 1,591  
Unrealized Gains 108  
Unrealized (Losses) (33)  
Fair Value 1,666  
Auction Rate Securities
    
Schedule of Available-for-sale Securities [Line Items]    
Fair Value   4,330
Auction Rate Securities | Long Term Investments
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost   4,925
Unrealized (Losses)   (595)
Fair Value   $ 4,330