Quarterly report pursuant to Section 13 or 15(d)

Marketable Securities (Narrative) (Details)

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Marketable Securities (Narrative) (Details) (USD $)
9 Months Ended 12 Months Ended
Jan. 31, 2012
Apr. 30, 2011
Schedule Of Marketable Securities [Line Items]    
Obligations for which assets are held in trust $ 80,700,000 $ 72,100,000
Trading securities 80,500,000 71,363,000
Non-current marketable securities, Trading 72,934,000 66,282,000
Available for sale marketable securities, maturities period, minimum, in months one  
Available for sale marketable securities, maturities period, maximum, in years 2.3  
Marketable Securities [Member]
   
Schedule Of Marketable Securities [Line Items]    
Gross unrealized gains 1,700,000 6,800,000
Gross unrealized losses $ 1,100,000 $ 100,000