Summary of Interest Expense Related to Convertible Notes (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jan. 31, 2018	Jan. 31, 2017	Jan. 31, 2018	Jan. 31, 2017
Interest Expense, Debt [Line Items]
Debt instrument, interest expense			$ 2,117	$ 1,940
5.25% Convertible Senior Notes Due 2019
Interest Expense, Debt [Line Items]
Debt instrument, interest expense	$ 2,084	$ 1,976	4,131	3,935
5.25% Convertible Senior Notes Due 2019 | Coupon Interest
Interest Expense, Debt [Line Items]
Debt instrument, interest expense	913	888	1,827	1,823
5.25% Convertible Senior Notes Due 2019 | Accretion of Debt Discount
Interest Expense, Debt [Line Items]
Debt instrument, interest expense	1,076	999	2,117	1,940
5.25% Convertible Senior Notes Due 2019 | Amortization of Debt Issue Cost
Interest Expense, Debt [Line Items]
Debt instrument, interest expense	$ 95	$ 89	$ 187	$ 172