Summary of Cash, Cash Equivalent and Marketable Securities by Significant Investment Categories (Detail) - USD ($)	Jul. 31, 2017	Jul. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable Securities		$ 0
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost	$ 174,300,000	118,700,000
Unrealized Losses	(10,000)
Fair Value	174,300,000	118,700,000
Cash and Cash Equivalents	129,300,000	118,700,000
Marketable Securities	45,000,000
Fair Value, Measurements, Recurring | Cash
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost	46,700,000	52,700,000
Fair Value	46,700,000	52,700,000
Cash and Cash Equivalents	46,700,000	52,700,000
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost	85,100,000
Fair Value	85,100,000
Cash and Cash Equivalents	85,100,000
Fair Value, Measurements, Recurring | Level 1 | Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost	38,400,000	66,000,000
Fair Value	38,400,000	66,000,000
Cash and Cash Equivalents	38,400,000	$ 66,000,000
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost	89,200,000
Unrealized Losses	(10,000)
Fair Value	89,200,000
Cash and Cash Equivalents	44,200,000
Marketable Securities	45,000,000
Fair Value, Measurements, Recurring | Level 2 | Non-U.S. government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost	3,900,000
Fair Value	3,900,000
Marketable Securities	3,900,000
Fair Value, Measurements, Recurring | Level 2 | Corporate Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost	28,000,000
Unrealized Losses	(10,000)
Fair Value	28,000,000
Marketable Securities	28,000,000
Fair Value, Measurements, Recurring | Level 2 | Asset-Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost	10,200,000
Fair Value	10,200,000
Marketable Securities	10,200,000
Fair Value, Measurements, Recurring | Level 2 | U.S. Treasury Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost	10,000,000
Fair Value	10,000,000
Cash and Cash Equivalents	10,000,000
Fair Value, Measurements, Recurring | Level 2 | U.S. Agency Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost	7,000,000
Fair Value	7,000,000
Cash and Cash Equivalents	7,000,000
Fair Value, Measurements, Recurring | Level 2 | Commercial Paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost	30,100,000
Fair Value	30,100,000
Cash and Cash Equivalents	27,200,000
Marketable Securities	$ 2,900,000