Short-Term Investments - Schedule of Fair Values and Gross Unrealized Losses of Available-for-Sale Securities Aggregated by Investment Category (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Oct. 31, 2018	Jan. 31, 2018
Certificates of deposits [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	$ 999
Gross unrealized losses	(1)
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	77,636	$ 65,690
Gross unrealized losses	(502)	(424)
Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	4,968	19,914
Gross unrealized losses		(12)
Corporate notes and bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	144,369	155,419
Gross unrealized losses	(758)	(759)
Foreign government bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	2,981	1,485
Gross unrealized losses	(20)	(18)
Mortgage backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	5,638	11,481
Gross unrealized losses	(14)	(75)
U.S. agency obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	35,723	66,655
Gross unrealized losses	(21)	(76)
U.S. treasury securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	248,754	82,147
Gross unrealized losses	$ (217)	$ (136)