Short-Term Investments - Schedule of Fair Values and Gross Unrealized Losses of Available-for-Sale Securities Aggregated by Investment Category (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Apr. 30, 2018	Jan. 31, 2018
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	$ 67,441	$ 65,690
Gross unrealized losses	(615)	(424)
Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	16,312	19,914
Gross unrealized losses	(9)	(12)
Corporate notes and bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	130,825	155,419
Gross unrealized losses	(1,011)	(759)
Foreign government bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	1,480	1,485
Gross unrealized losses	(23)	(18)
Mortgage backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	11,425	11,481
Gross unrealized losses	(100)	(75)
U.S. agency obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	58,526	66,655
Gross unrealized losses	(59)	(76)
U.S. treasury securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	113,148	82,147
Gross unrealized losses	$ (218)	$ (136)