Short-Term Investments - Schedule of Short-Term Investments (Detail) - USD ($) $ in Thousands	Apr. 30, 2019	Jan. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Amortized cost	$ 581,089	$ 539,743
Available-for-sale securities, Gross unrealized gains	761	282
Available-for-sale securities, Gross unrealized losses	(360)	(835)
Available-for-sale securities, Estimated fair value	581,490	539,190
Certificates of deposits [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Amortized cost	3,500	6,001
Available-for-sale securities, Gross unrealized gains	11	10
Available-for-sale securities, Gross unrealized losses		(1)
Available-for-sale securities, Estimated fair value	3,511	6,010
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Amortized cost	81,578	78,682
Available-for-sale securities, Gross unrealized gains	65	13
Available-for-sale securities, Gross unrealized losses	(138)	(300)
Available-for-sale securities, Estimated fair value	81,505	78,395
Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Amortized cost	13,613	9,118
Available-for-sale securities, Gross unrealized gains	6	1
Available-for-sale securities, Gross unrealized losses		(2)
Available-for-sale securities, Estimated fair value	13,619	9,117
Corporate notes and bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Amortized cost	226,957	185,409
Available-for-sale securities, Gross unrealized gains	526	178
Available-for-sale securities, Gross unrealized losses	(206)	(457)
Available-for-sale securities, Estimated fair value	227,277	185,130
Foreign government bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Amortized cost	1,501	1,502
Available-for-sale securities, Gross unrealized losses	(8)	(11)
Available-for-sale securities, Estimated fair value	1,493	1,491
U.S. agency obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Amortized cost	3,000	15,912
Available-for-sale securities, Gross unrealized gains	1	2
Available-for-sale securities, Gross unrealized losses		(2)
Available-for-sale securities, Estimated fair value	3,001	15,912
U.S. treasury securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Amortized cost	250,940	243,119
Available-for-sale securities, Gross unrealized gains	152	78
Available-for-sale securities, Gross unrealized losses	(8)	(62)
Available-for-sale securities, Estimated fair value	$ 251,084	$ 243,135