Assets and Liabilities Carried at Fair Value and Measured on Recurring Basis (Detail) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Oct. 31, 2017	Jul. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost	$ 182,900	$ 174,300
Unrealized Losses	(50)	(10)
Fair Value	182,900	174,300
Cash and Cash Equivalents	96,800	129,300
Marketable Securities	86,100	45,000
Cash
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost	43,400	46,700
Fair Value	43,400	46,700
Cash and Cash Equivalents	43,400	46,700
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost	75,800	85,100
Fair Value	75,800	85,100
Cash and Cash Equivalents	75,800	85,100
Level 1 | Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost	32,400	38,400
Fair Value	32,400	38,400
Cash and Cash Equivalents	32,400	38,400
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost	107,100	89,200
Unrealized Losses	(50)	(10)
Fair Value	107,100	89,200
Cash and Cash Equivalents	21,000	44,200
Marketable Securities	86,100	45,000
Level 2 | Non-U.S. government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost	10,500	3,900
Fair Value	10,500	3,900
Marketable Securities	10,500	3,900
Level 2 | Corporate Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost	40,400	28,000
Unrealized Losses	(30)	(10)
Fair Value	40,400	28,000
Marketable Securities	40,400	28,000
Level 2 | Asset-Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost	24,500	10,200
Unrealized Losses	(20)
Fair Value	24,500	10,200
Marketable Securities	24,500	10,200
Level 2 | U.S. Agency Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost	4,000	7,000
Fair Value	4,000	7,000
Cash and Cash Equivalents		7,000
Marketable Securities	4,000
Level 2 | Commercial Paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost	27,700	30,100
Fair Value	27,700	30,100
Cash and Cash Equivalents	21,000	27,200
Marketable Securities	$ 6,700	2,900
Level 2 | US Treasury Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost		10,000
Fair Value		10,000
Cash and Cash Equivalents		$ 10,000