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Fair Value Of Financial Instruments - Additional Information (Detail) (USD $)
6 Months Ended
Oct. 31, 2012
Oct. 31, 2012
Redeemed Available-for-Sale Securities
Oct. 31, 2011
Redeemed Available-for-Sale Securities
Schedule of Available-for-sale Securities [Line Items]      
Redeemed available-for-sale securities   $ 2,000,000 $ 5,100,000
Realized gain $ 7,000