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, 2016 USD ($)
Schedule Of Available For Sale Securities [Line Items]
Fair Value of Less Than 12 Months	$ 57,091
Unrealized Losses of Less Than 12 Months	(34)
U.S. Government Notes
Schedule Of Available For Sale Securities [Line Items]
Fair Value of Less Than 12 Months	28,883
Unrealized Losses of Less Than 12 Months	(6)
U.S. Government Agencies
Schedule Of Available For Sale Securities [Line Items]
Fair Value of Less Than 12 Months	2,995
Unrealized Losses of Less Than 12 Months	(1)
Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Fair Value of Less Than 12 Months	25,213
Unrealized Losses of Less Than 12 Months	$ (27)