Assets and Liabilities Carried at Fair Value and Measured on Recurring Basis (Detail) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Apr. 30, 2018	Jul. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost	$ 222.4	$ 174.3
Unrealized Losses	(0.1)
Fair Value	222.3	174.3
Cash and Cash Equivalents	123.9	129.3
Marketable Securities	98.4	45.0
Cash
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost	103.6	46.7
Fair Value	103.6	46.7
Cash and Cash Equivalents	103.6	46.7
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost	112.6	85.1
Fair Value	112.6	85.1
Cash and Cash Equivalents	112.6	85.1
Level 1 | Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost	9.0	38.4
Fair Value	9.0	38.4
Cash and Cash Equivalents	9.0	38.4
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost	109.8	89.2
Unrealized Losses	(0.1)
Fair Value	109.7	89.2
Cash and Cash Equivalents	11.3	44.2
Marketable Securities	98.4	45.0
Level 2 | Non-U.S. government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost	9.7	3.9
Fair Value	9.7	3.9
Marketable Securities	9.7	3.9
Level 2 | Corporate Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost	53.1	28.0
Fair Value	53.1	28.0
Cash and Cash Equivalents	0.5
Marketable Securities	52.6	28.0
Level 2 | Asset-Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost	21.6	10.2
Unrealized Losses	(0.1)
Fair Value	21.5	10.2
Marketable Securities	21.5	10.2
Level 2 | U.S. Agency Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost	4.0	7.0
Fair Value	4.0	7.0
Cash and Cash Equivalents		7.0
Marketable Securities	4.0
Level 2 | Commercial Paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost	21.4	30.1
Fair Value	21.4	30.1
Cash and Cash Equivalents	10.8	27.2
Marketable Securities	$ 10.6	2.9
Level 2 | US Treasury Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted Cost		10.0
Fair Value		10.0
Cash and Cash Equivalents		$ 10.0