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Fair Value of Option Grant Assumptions (Parenthetical) (Detail) (Total Shareholders Return)
3 Months Ended 6 Months Ended
Jan. 31, 2013
Jan. 31, 2013
Jan. 31, 2012
Total Shareholders Return
     
Share based Compensation Arrangement by Share based Payment Award, Fair Value Assumptions, Method Used [Line Items]      
Expected annual dividend yield 0.00% 0.00% [1] 0.00% [1]
[1] Dividends are considered reinvested when calculating TSR. For the purpose of the fair value model, the dividend yield is therefore considered to be 0%.