Financial Instruments - Gross Unrealized Losses and Fair Values (Details) $ in Thousands	Jul. 31, 2019 USD ($)
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Fair Value, Less than 12 months	$ 81,357
Unrealized Loss, Less than 12 Months	(76)
Fair Value, Greater than 12 months	62,165
Unrealized Loss, Greater than 12 months	(126)
Fair Value, Total	143,522
Unrealized Loss, Total	(202)
U.S. government treasury notes
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Fair Value, Less than 12 months	27,887
Unrealized Loss, Less than 12 Months	(30)
Fair Value, Greater than 12 months	9,737
Unrealized Loss, Greater than 12 months	(28)
Fair Value, Total	37,624
Unrealized Loss, Total	(58)
U.S. government agencies
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Fair Value, Less than 12 months	5,215
Unrealized Loss, Less than 12 Months	(7)
Fair Value, Greater than 12 months	9,183
Unrealized Loss, Greater than 12 months	(15)
Fair Value, Total	14,398
Unrealized Loss, Total	(22)
Corporate debt securities
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Fair Value, Less than 12 months	38,856
Unrealized Loss, Less than 12 Months	(28)
Fair Value, Greater than 12 months	43,245
Unrealized Loss, Greater than 12 months	(83)
Fair Value, Total	82,101
Unrealized Loss, Total	(111)
Asset-backed securities
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Fair Value, Less than 12 months	9,399
Unrealized Loss, Less than 12 Months	(11)
Fair Value, Greater than 12 months	0
Unrealized Loss, Greater than 12 months	0
Fair Value, Total	9,399
Unrealized Loss, Total	$ (11)