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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
Apr. 30, 2014
Jan. 31, 2014
Apr. 30, 2013
Schedule of Available-for-sale Securities [Line Items]      
Amortized Cost $ 325,082 $ 647,483 $ 364,396
Unrealized Gains 483 1,002 389
Unrealized (Losses) (75) (250) (76)
Fair Value 325,490 648,235 364,709
Short-term Investments
      
Schedule of Available-for-sale Securities [Line Items]      
Amortized Cost 149,724 281,674 214,828
Unrealized Gains 90 184 137
Unrealized (Losses) (18) (45) (27)
Fair Value 149,796 281,813 214,938
Long Term Investments
      
Schedule of Available-for-sale Securities [Line Items]      
Amortized Cost 175,358 365,809 149,568
Unrealized Gains 393 818 252
Unrealized (Losses) (57) (205) (49)
Fair Value 175,694 366,422 149,771
Corporate Bonds
      
Schedule of Available-for-sale Securities [Line Items]      
Fair Value 140,411 309,423 153,376
Corporate Bonds | Short-term Investments
      
Schedule of Available-for-sale Securities [Line Items]      
Amortized Cost 54,678 100,856 76,563
Unrealized Gains 30 56 49
Unrealized (Losses) (17) (41) (26)
Fair Value 54,691 100,871 76,586
Corporate Bonds | Long Term Investments
      
Schedule of Available-for-sale Securities [Line Items]      
Amortized Cost 85,583 208,446 76,717
Unrealized Gains 168 268 117
Unrealized (Losses) (31) (162) (44)
Fair Value 85,720 208,552 76,790
Municipal And Pre-Refunded Municipal Bonds
      
Schedule of Available-for-sale Securities [Line Items]      
Fair Value 113,238 211,437 126,594
Municipal And Pre-Refunded Municipal Bonds | Short-term Investments
      
Schedule of Available-for-sale Securities [Line Items]      
Amortized Cost 44,522 85,000 75,507
Unrealized Gains 41 98 58
Unrealized (Losses)   (2) (1)
Fair Value 44,563 85,096 75,564
Municipal And Pre-Refunded Municipal Bonds | Long Term Investments
      
Schedule of Available-for-sale Securities [Line Items]      
Amortized Cost 68,511 125,934 50,913
Unrealized Gains 178 415 121
Unrealized (Losses) (14) (8) (4)
Fair Value 68,675 126,341 51,030
Certificates of Deposit
      
Schedule of Available-for-sale Securities [Line Items]      
Fair Value 24,857 39,854 42,345
Certificates of Deposit | Short-term Investments
      
Schedule of Available-for-sale Securities [Line Items]      
Amortized Cost 23,348 35,844 40,222
Unrealized Gains 10 13 23
Unrealized (Losses) (1) (1)  
Fair Value 23,357 35,856 40,245
Certificates of Deposit | Long Term Investments
      
Schedule of Available-for-sale Securities [Line Items]      
Amortized Cost 1,499 4,000 2,099
Unrealized Gains 1   1
Unrealized (Losses)   (2)  
Fair Value 1,500 3,998 2,100
Commercial Paper
      
Schedule of Available-for-sale Securities [Line Items]      
Fair Value 24,684 35,107 8,482
Commercial Paper | Short-term Investments
      
Schedule of Available-for-sale Securities [Line Items]      
Amortized Cost 24,678 35,101 8,476
Unrealized Gains 6 7 6
Unrealized (Losses)   (1)  
Fair Value 24,684 35,107 8,482
Treasury Bills
      
Schedule of Available-for-sale Securities [Line Items]      
Fair Value 17,914 46,455 21,731
Treasury Bills | Short-term Investments
      
Schedule of Available-for-sale Securities [Line Items]      
Amortized Cost 2,498 24,873 12,060
Unrealized Gains 3 10 1
Fair Value 2,501 24,883 12,061
Treasury Bills | Long Term Investments
      
Schedule of Available-for-sale Securities [Line Items]      
Amortized Cost 15,404 21,551 9,666
Unrealized Gains 9 21 4
Fair Value 15,413 21,572 9,670
Mutual Funds, Held in Rabbi Trust
      
Schedule of Available-for-sale Securities [Line Items]      
Fair Value 2,600 1,666 337
Mutual Funds, Held in Rabbi Trust | Long Term Investments
      
Schedule of Available-for-sale Securities [Line Items]      
Amortized Cost 2,579 1,591 328
Unrealized Gains 33 108 9
Unrealized (Losses) (12) (33)  
Fair Value 2,600 1,666 337
Federal Government Agencies
      
Schedule of Available-for-sale Securities [Line Items]      
Fair Value 1,786 4,293 11,844
Federal Government Agencies | Short-term Investments
      
Schedule of Available-for-sale Securities [Line Items]      
Amortized Cost     2,000
Fair Value     2,000
Federal Government Agencies | Long Term Investments
      
Schedule of Available-for-sale Securities [Line Items]      
Amortized Cost 1,782 4,287 9,845
Unrealized Gains 4 6  
Unrealized (Losses)     (1)
Fair Value $ 1,786 $ 4,293 $ 9,844