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Marketable Securities (Narrative) (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Apr. 30, 2012
Apr. 30, 2011
Jan. 31, 2012
Schedule of Available-for-sale Securities [Line Items]      
Proceeds from sale and maturities of available-for-sale securities $ 53,829 $ 115,442  
Amortization of discounts and premiums, net 1,235 2,322  
Estimated fair value of available-for-sale securities 208,287 402,634 216,767
Other Income [Member]
     
Schedule of Available-for-sale Securities [Line Items]      
Net realized gains 127 28  
Auction Rate Securities [Member]
     
Schedule of Available-for-sale Securities [Line Items]      
Par value of available-for-sale securities 4,925    
Estimated fair value of available-for-sale securities $ 4,330 $ 26,716 $ 20,197