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Marketable Securities - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
3 Months Ended 6 Months Ended
Jul. 31, 2012
Jul. 31, 2011
Jul. 31, 2012
Jul. 31, 2011
Jan. 31, 2012
Schedule of Available-for-sale Securities [Line Items]          
Proceeds from sale and maturities of available-for-sale securities     $ 122,343 $ 157,467  
Amortization of discounts and premiums, net 1,243 2,007 2,478 4,329  
Estimated fair value of available-for-sale securities 227,554 382,249 227,554 382,249 216,767
Auction Rate Securities
         
Schedule of Available-for-sale Securities [Line Items]          
Par value of available-for-sale securities 4,925   4,925    
Estimated fair value of available-for-sale securities 4,330 23,504 4,330 23,504 20,197
Other Income
         
Schedule of Available-for-sale Securities [Line Items]          
Net realized gains $ 85 $ 23 $ 212 $ 51