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	12 Months Ended
	Jan. 31, 2017 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Fair Value	$ 217,300
Unrealized Loss	(830)
Gross realized gains, available-for-sale securities	253
U.S. government treasury notes
Schedule of Available-for-sale Securities [Line Items]
Fair Value	82,228
Unrealized Loss	(289)
U.S. government agencies
Schedule of Available-for-sale Securities [Line Items]
Fair Value	34,485
Unrealized Loss	(159)
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value	99,710
Unrealized Loss	(379)
Foreign government bonds
Schedule of Available-for-sale Securities [Line Items]
Fair Value	877
Unrealized Loss	$ (3)