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Fair Value of TSR Performance-Based Awards Valuation Assumption (Detail) - Total Shareholders Return Performance-based Awards - $ / shares
3 Months Ended 9 Months Ended
Apr. 30, 2018
Apr. 30, 2017
Apr. 30, 2018
Apr. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock price [1]   $ 73.60 $ 77.15 $ 89.88
Expected volatility [2] 0.00% 27.00% 26.88% 27.00%
Risk-free interest rate [3] 0.00% 0.85% 1.48% 0.85%
Expected annual dividend yield [4] 0.00% 0.00% 0.00% 0.00%
Time Based Restricted Stock Awards        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Weighted average grant date fair value of time-based restricted stock awards $ 85.65 $ 78.22 $ 78.16 $ 88.16
Performance Based Restricted Stock Awards        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Weighted average grant date fair value of time-based restricted stock awards   $ 73.60 $ 79.87 $ 86.88
[1] The stock price is the closing price of our common stock on the date of grant.
[2] The expected volatility for each grant is determined based on the historical volatility for the peer group companies and our common stock over a period equal to the remaining term of the performance period from the date of grant for all awards.
[3] The risk-free interest rate is determined based on the yield of zero-coupon U.S. Treasury securities for a period that is commensurate with the performance period.
[4] Dividends are considered reinvested when calculating TSR. The dividend yield is therefore considered to be 0%.