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, 2017	Jan. 31, 2017
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	$ 63,898	$ 14,027
Gross unrealized losses	(126)	(16)
Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	9,749	5,694
Gross unrealized losses	(6)	(1)
Corporate notes and bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	151,950	67,220
Gross unrealized losses	(215)	(101)
Foreign government bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	1,497
Gross unrealized losses	(7)
Mortgage backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	13,142
Gross unrealized losses	(32)
U.S. agency obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	73,500	40,549
Gross unrealized losses	(67)	(51)
U.S. treasury securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	88,644	68,704
Gross unrealized losses	$ (106)	$ (111)