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, 2018 USD ($)
Fair Value
Less than 12 months	$ 229,033
Greater than 12 months	97,577
Total	326,610
Unrealized Loss
Less than 12 months	(1,701)
Greater than 12 months	(1,009)
Total	(2,710)
U.S. government treasury notes
Fair Value
Less than 12 months	93,195
Greater than 12 months	40,159
Total	133,354
Unrealized Loss
Less than 12 months	(323)
Greater than 12 months	(451)
Total	(774)
U.S. government agencies
Fair Value
Less than 12 months	16,216
Greater than 12 months	24,069
Total	40,285
Unrealized Loss
Less than 12 months	(190)
Greater than 12 months	(200)
Total	(390)
Corporate debt securities
Fair Value
Less than 12 months	119,622
Greater than 12 months	33,349
Total	152,971
Unrealized Loss
Less than 12 months	(1,188)
Greater than 12 months	(358)
Total	$ (1,546)