Fair Value of Financial Instruments (Details 3) - USD ($) $ in Thousands	Jan. 31, 2018	Jul. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Less than 12 Months	$ 456,267
12 Months or Greater	64,273
Estimated Fair Value	520,540	$ 619,099
U.S. Government agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Less than 12 Months	19,588
12 Months or Greater	1,730
Estimated Fair Value	21,318	22,596
Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Less than 12 Months	99,966
12 Months or Greater	0
Estimated Fair Value	99,966	147,339
Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Less than 12 Months	174,084
12 Months or Greater	56,052
Estimated Fair Value	230,136	258,345
U.S. government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Less than 12 Months	50,001
12 Months or Greater	0
Estimated Fair Value	50,001	66,979
Foreign government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value		27,527
Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Less than 12 Months	92,380
12 Months or Greater	0
Estimated Fair Value	92,380	$ 96,313
Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Less than 12 Months	20,248
12 Months or Greater	6,491
Estimated Fair Value	$ 26,739