Assets and Liabilities Carried at Fair Value and Measured on Recurring Basis (Detail) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Jan. 31, 2018	Jul. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost	$ 202,400	$ 174,300
Unrealized Losses	(100)	(10)
Fair Value	202,300	174,300
Cash and Cash Equivalents	101,900	129,300
Marketable Securities	100,400	45,000
Cash
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost	53,500	46,700
Fair Value	53,500	46,700
Cash and Cash Equivalents	53,500	46,700
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost	87,700	85,100
Fair Value	87,700	85,100
Cash and Cash Equivalents	87,700	85,100
Level 1 | Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost	34,200	38,400
Fair Value	34,200	38,400
Cash and Cash Equivalents	34,200	38,400
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost	114,700	89,200
Unrealized Losses	(100)	(10)
Fair Value	114,600	89,200
Cash and Cash Equivalents	14,200	44,200
Marketable Securities	100,400	45,000
Level 2 | Non-U.S. government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost	11,500	3,900
Fair Value	11,500	3,900
Marketable Securities	11,500	3,900
Level 2 | Corporate Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost	47,300	28,000
Unrealized Losses		(10)
Fair Value	47,300	28,000
Marketable Securities	47,300	28,000
Level 2 | Asset-Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost	28,300	10,200
Unrealized Losses	(100)
Fair Value	28,200	10,200
Cash and Cash Equivalents	1,000
Marketable Securities	27,200	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	23,600	30,100
Fair Value	23,600	30,100
Cash and Cash Equivalents	13,200	27,200
Marketable Securities	$ 10,400	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