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	3 Months Ended	9 Months Ended
	Oct. 31, 2016 USD ($)	Oct. 31, 2016 USD ($)
Schedule Of Available For Sale Securities [Line Items]
Fair Value of Less Than 12 Months	$ 111,404	$ 111,404
Unrealized Losses of Less Than 12 Months	(192)	(192)
Gross realized gains, available-for-sale securities	65	175
U.S. government treasury notes
Schedule Of Available For Sale Securities [Line Items]
Fair Value of Less Than 12 Months	35,647	35,647
Unrealized Losses of Less Than 12 Months	(27)	(27)
U.S. government agencies
Schedule Of Available For Sale Securities [Line Items]
Fair Value of Less Than 12 Months	28,274	28,274
Unrealized Losses of Less Than 12 Months	(55)	(55)
Corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Fair Value of Less Than 12 Months	47,483	47,483
Unrealized Losses of Less Than 12 Months	$ (110)	$ (110)