Short-Term Investments - Schedule of Fair Values and Gross Unrealized Losses of Available-for-Sale Securities Aggregated by Investment Category (Detail) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jul. 31, 2018	Jan. 31, 2018
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	$ 82,019	$ 65,690
Gross unrealized losses	(555)	(424)
Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	3,892	19,914
Gross unrealized losses	(1)	(12)
Corporate notes and bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	108,287	155,419
Gross unrealized losses	(718)	(759)
Foreign government bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	2,979	1,485
Gross unrealized losses	(21)	(18)
Mortgage backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	13,285	11,481
Gross unrealized losses	(81)	(75)
U.S. agency obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	49,893	66,655
Gross unrealized losses	(25)	(76)
U.S. treasury securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	153,505	82,147
Gross unrealized losses	$ (204)	$ (136)