Financial Instruments - Schedule of Amortized Cost and Estimated Fair Value of Marketable Securities by Contractual Maturity (Details) (Details) $ in Thousands	Jan. 31, 2019 USD ($)
Fair Value Disclosures [Abstract]
Due within one year, Amortized Cost	$ 342,739
Due in one to five years, Amortized Cost	407,081
Total, Amortized Cost	749,820
Due within one year, Fair Value	342,256
Due in one to five years, Fair Value	407,226
Total, Fair Value	$ 749,482