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	Oct. 31, 2017 USD ($)
Fair Value
Less than 12 months	$ 235,732
Greater than 12 months	41,304
Total	277,036
Unrealized Loss
Less than 12 months	(655)
Greater than 12 months	(279)
Total	(934)
U.S. government treasury notes
Fair Value
Less than 12 months	122,530
Greater than 12 months	3,965
Total	126,495
Unrealized Loss
Less than 12 months	(381)
Greater than 12 months	(38)
Total	(419)
U.S. government agencies
Fair Value
Less than 12 months	31,914
Greater than 12 months	12,398
Total	44,312
Unrealized Loss
Less than 12 months	(106)
Greater than 12 months	(99)
Total	(205)
Corporate debt securities
Fair Value
Less than 12 months	81,288
Greater than 12 months	24,941
Total	106,229
Unrealized Loss
Less than 12 months	(168)
Greater than 12 months	(142)
Total	$ (310)