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	Jan. 31, 2018 USD ($)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	$ 208,381
Greater than 12 months	109,631
Total	318,012
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Less than 12 months	(1,107)
Greater than 12 months	(926)
Total	(2,033)
U.S. government treasury notes
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	68,212
Greater than 12 months	52,607
Total	120,819
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Less than 12 months	(219)
Greater than 12 months	(432)
Total	(651)
U.S. government agencies
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	23,004
Greater than 12 months	23,892
Total	46,896
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Less than 12 months	(156)
Greater than 12 months	(177)
Total	(333)
Corporate debt securities
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	117,165
Greater than 12 months	33,132
Total	150,297
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Less than 12 months	(732)
Greater than 12 months	(317)
Total	$ (1,049)