Summary of Performance Based RSU Awards (Detail) - $ / shares									6 Months Ended		12 Months Ended
		Dec. 05, 2017	Apr. 28, 2017	Dec. 08, 2016	Oct. 21, 2016	Mar. 10, 2016	Jan. 29, 2016	Jan. 19, 2016	Jan. 31, 2018	Jan. 31, 2017	Jul. 31, 2017	Jul. 31, 2016
Series Award Four | Tranche One
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected Percentage of Vesting		100.00%									100.00%
Series Award Four | Second Tranche
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected Percentage of Vesting		172.00%									184.00%
Series Award Three
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected Percentage of Vesting		100.00%										151.00%
Series Award One
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected Percentage of Vesting											184.00%	196.00%
Performance-based Restricted Stock Unit ("RSU") Awards
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Target Award Shares									9,877			58,805
Target Award Shares, forfeited	[1]			(654)	(4,957)		(12,983)
Grant Date Fair Value									$ 54.09	$ 29.64
Shares Expected to Vest												115,503
Performance-based Restricted Stock Unit ("RSU") Awards | Series Award Four | Tranche One
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Grant Dates		Dec. 05, 2017	Apr. 28, 2017		Oct. 21, 2016
Target Award Shares		27,541	4,545		44,337				43,925
Grant Date Fair Value		$ 54.09	$ 52.55		$ 29.11
Measurement Period Ending		Jul. 31, 2020	Jul. 31, 2019		Jul. 31, 2019
Expected Percentage of Vesting	[2]	100.00%							100.00%
Shares Expected to Vest		27,541							43,925
Performance-based Restricted Stock Unit ("RSU") Awards | Series Award Four | Second Tranche
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Grant Dates		Dec. 05, 2017	Apr. 28, 2017		Oct. 21, 2016
Target Award Shares		27,542	4,546		44,337				43,926
Grant Date Fair Value		$ 54.09	$ 52.55		$ 29.11
Measurement Period Ending		Jul. 31, 2020	Jul. 31, 2019		Jul. 31, 2019
Expected Percentage of Vesting	[2]	172.00%							184.00%
Shares Expected to Vest		47,371							80,986
Performance-based Restricted Stock Unit ("RSU") Awards | Series Award Three
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Grant Dates		Dec. 05, 2017						Jan. 19, 2016
Target Award Shares		14,000						82,938
Grant Date Fair Value		$ 54.09						$ 20.89
Measurement Period Ending		Jul. 31, 2018						Jul. 31, 2020
Expected Percentage of Vesting	[2]	100.00%						151.00%
Shares Expected to Vest		14,000						125,236
Performance-based Restricted Stock Unit ("RSU") Awards | Series Award One
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Grant Dates				Dec. 08, 2016		Mar. 10, 2016	Jan. 29, 2016
Target Award Shares				10,531		14,625	57,163
Grant Date Fair Value						$ 21.89	$ 21.80		$ 34.95
Measurement Period Ending						Oct. 31, 2018	Oct. 31, 2018		Jul. 31, 2019
Expected Percentage of Vesting	[2]								184.00%			196.00%
Shares Expected to Vest									18,210

[1] Forfeitures include Series 1 and Series 4 awards that were granted to certain employees in fiscal years 2017 and 2016 but that were forfeited at the termination of their employment

[2] The percentage vesting for performance-based RSU awards is currently estimated at 184% and 196% of the target award for Series 1 awards granted in fiscal years 2017 and 2016, respectively, 100% and 151% of the target award for Series 3 awards granted in fiscal years 2018 and 2016, respectively, 100% and 172% of the target award for the first and second tranches, respectively, of the Series 4 awards granted in fiscal year 2018, and 100% and 184% of the target award for the first and second tranches, respectively, of the Series 4 awards granted in fiscal year 2017.