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	Apr. 30, 2017 USD ($)
Fair Value
Less than 12 months	$ 245,380
Greater than 12 months	2,269
Total	247,649
Unrealized Loss
Less than 12 months	(763)
Greater than 12 months	(6)
Total	(769)
U.S. government treasury notes
Fair Value
Less than 12 months	120,441
Greater than 12 months	0
Total	120,441
Unrealized Loss
Less than 12 months	(313)
Greater than 12 months	0
Total	(313)
U.S. government agencies
Fair Value
Less than 12 months	38,679
Greater than 12 months	0
Total	38,679
Unrealized Loss
Less than 12 months	(147)
Greater than 12 months	0
Total	(147)
Corporate debt securities
Fair Value
Less than 12 months	85,382
Greater than 12 months	2,269
Total	87,651
Unrealized Loss
Less than 12 months	(301)
Greater than 12 months	(6)
Total	(307)
Foreign government bonds
Fair Value
Less than 12 months	878
Greater than 12 months	0
Total	878
Unrealized Loss
Less than 12 months	(2)
Greater than 12 months	0
Total	$ (2)