Fair Value of TSR Performance-Based Awards Valuation Assumption (Detail) - Total Shareholders Return Performance-based Awards - $ / shares		3 Months Ended		6 Months Ended
		Jan. 31, 2018	Jan. 31, 2017	Jan. 31, 2018	Jan. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock price	[1]			$ 77.15	$ 90.23
Expected volatility	[2]	0.00%	0.00%	26.88%	27.00%
Risk-free interest rate	[3]	0.00%	0.00%	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		$ 82.71	$ 92.30	$ 77.19	$ 89.48
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		$ 0.00		$ 79.87	$ 87.05

[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%.