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Fair Value (Narrative) (Details) (Auction Rate Securities [Member], USD $)
In Thousands, unless otherwise specified
 3 Months Ended 12 Months Ended
Apr. 30, 2012
Apr. 30, 2011
Jan. 31, 2012
Auction Rate Securities [Member]
      
Schedule of Available-for-sale Securities [Line Items]      
Temporary impairment of auction rate securities $ 595 $ 3,434 $ 2,778
Hypothetical decline in fair value of available-for-sale securities, basis points 100    
Hypothetical decline in fair value of available-for-sale securities, temporary impairment amount $ 645