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) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jul. 31, 2016	Jul. 31, 2016	Jul. 31, 2017
Fair Value
Less than 12 months			$ 220,635
Greater than 12 months			10,728
Total			231,363
Unrealized Loss
Less than 12 months			(605)
Greater than 12 months			(58)
Total			(663)
Realized gain (loss)	$ 104	$ 110
U.S. government treasury notes
Fair Value
Less than 12 months			121,518
Greater than 12 months			1,986
Total			123,504
Unrealized Loss
Less than 12 months			(299)
Greater than 12 months			(18)
Total			(317)
U.S. government agencies
Fair Value
Less than 12 months			35,916
Greater than 12 months			4,471
Total			40,387
Unrealized Loss
Less than 12 months			(112)
Greater than 12 months			(27)
Total			(139)
Corporate debt securities
Fair Value
Less than 12 months			62,323
Greater than 12 months			4,271
Total			66,594
Unrealized Loss
Less than 12 months			(192)
Greater than 12 months			(13)
Total			(205)
Foreign government bonds
Fair Value
Less than 12 months			878
Greater than 12 months			0
Total			878
Unrealized Loss
Less than 12 months			(2)
Greater than 12 months			0
Total			$ (2)