Financial Instruments - Schedule of Gross Unrealized Losses and Fair Values for Investments that were in Continuous Unrealized Loss Position for Less Than 12 Months, Aggregated by Investments Category (Details) $ in Thousands	Jul. 31, 2018 USD ($)
Fair Value
Less than 12 months	$ 607,179
Greater than 12 months	99,969
Total	707,148
Unrealized Loss
Less than 12 months	(2,169)
Greater than 12 months	(762)
Total	(2,931)
U.S. government treasury notes
Fair Value
Less than 12 months	271,578
Greater than 12 months	48,941
Total	320,519
Unrealized Loss
Less than 12 months	(483)
Greater than 12 months	(356)
Total	(839)
U.S. government agencies
Fair Value
Less than 12 months	55,695
Greater than 12 months	20,374
Total	76,069
Unrealized Loss
Less than 12 months	(227)
Greater than 12 months	(147)
Total	(374)
Corporate debt securities
Fair Value
Less than 12 months	271,946
Greater than 12 months	30,654
Total	302,600
Unrealized Loss
Less than 12 months	(1,454)
Greater than 12 months	(259)
Total	(1,713)
Foreign government bonds
Fair Value
Less than 12 months	2,944
Greater than 12 months	0
Total	2,944
Unrealized Loss
Less than 12 months	(3)
Greater than 12 months	0
Total	(3)
Asset-backed securities
Fair Value
Less than 12 months	5,016
Greater than 12 months	0
Total	5,016
Unrealized Loss
Less than 12 months	(2)
Greater than 12 months	0
Total	$ (2)