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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
Jul. 31, 2014
Jan. 31, 2014
Jul. 31, 2013
Schedule of Available-for-sale Securities [Line Items]      
Amortized Cost $ 263,674 $ 647,483 $ 442,558
Unrealized Gains 375 1,002 355
Unrealized (Losses) (182) (250) (348)
Fair Value 263,867 648,235 442,565
Short-term Investments
      
Schedule of Available-for-sale Securities [Line Items]      
Amortized Cost 106,641 281,674 225,725
Unrealized Gains 96 184 120
Unrealized (Losses) (16) (45) (46)
Fair Value 106,721 281,813 225,799
Long Term Investments
      
Schedule of Available-for-sale Securities [Line Items]      
Amortized Cost 157,033 365,809 216,833
Unrealized Gains 279 818 235
Unrealized (Losses) (166) (205) (302)
Fair Value 157,146 366,422 216,766
Corporate Bonds | Short-term Investments
      
Schedule of Available-for-sale Securities [Line Items]      
Amortized Cost 55,849 100,856 79,637
Unrealized Gains 50 56 22
Unrealized (Losses) (15) (41) (37)
Fair Value 55,884 100,871 79,622
Corporate Bonds | Long Term Investments
      
Schedule of Available-for-sale Securities [Line Items]      
Amortized Cost 93,086 208,446 95,795
Unrealized Gains 96 268 51
Unrealized (Losses) (143) (162) (239)
Fair Value 93,039 208,552 95,607
Municipal And Pre-Refunded Municipal Bonds | Short-term Investments
      
Schedule of Available-for-sale Securities [Line Items]      
Amortized Cost 29,022 85,000 91,819
Unrealized Gains 29 98 61
Unrealized (Losses) (1) (2) (9)
Fair Value 29,050 85,096 91,871
Municipal And Pre-Refunded Municipal Bonds | Long Term Investments
      
Schedule of Available-for-sale Securities [Line Items]      
Amortized Cost 45,387 125,934 100,402
Unrealized Gains 107 415 142
Unrealized (Losses) (18) (8) (54)
Fair Value 45,476 126,341 100,490
Certificates of Deposit | Short-term Investments
      
Schedule of Available-for-sale Securities [Line Items]      
Amortized Cost 14,348 35,844 29,870
Unrealized Gains 9 13 24
Unrealized (Losses)   (1)  
Fair Value 14,357 35,856 29,894
Certificates of Deposit | Long Term Investments
      
Schedule of Available-for-sale Securities [Line Items]      
Amortized Cost 2,493 4,000 3,801
Unrealized Gains     1
Unrealized (Losses)   (2) (2)
Fair Value 2,493 3,998 3,800
Treasury Bills | Short-term Investments
      
Schedule of Available-for-sale Securities [Line Items]      
Amortized Cost 4,530 24,873 16,115
Unrealized Gains 4 10 5
Fair Value 4,534 24,883 16,120
Treasury Bills | Long Term Investments
      
Schedule of Available-for-sale Securities [Line Items]      
Amortized Cost 11,264 21,551 8,218
Unrealized Gains 5 21 6
Fair Value 11,269 21,572 8,224
Commercial Paper | Short-term Investments
      
Schedule of Available-for-sale Securities [Line Items]      
Amortized Cost 2,892 35,101 8,284
Unrealized Gains 4 7 8
Unrealized (Losses)   (1)  
Fair Value 2,896 35,107 8,292
Mutual Funds, Held in Rabbi Trust | Long Term Investments
      
Schedule of Available-for-sale Securities [Line Items]      
Amortized Cost 3,026 1,591 817
Unrealized Gains 68 108 35
Unrealized (Losses) (5) (33)  
Fair Value 3,089 1,666 852
Federal Government Agencies | Long Term Investments
      
Schedule of Available-for-sale Securities [Line Items]      
Amortized Cost 1,777 4,287 7,800
Unrealized Gains 3 6  
Unrealized (Losses)     (7)
Fair Value $ 1,780 $ 4,293 $ 7,793