Short-Term Investments - Schedule of Short-Term Investments (Detail) - USD ($) $ in Thousands	Jul. 31, 2019	Jan. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 603,378	$ 539,743
Gross unrealized gains	1,509	282
Gross unrealized losses	(199)	(835)
Estimated fair value	604,688	539,190
Certificates of deposits
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	3,500	6,001
Gross unrealized gains	8	10
Gross unrealized losses	0	(1)
Estimated fair value	3,508	6,010
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	82,825	78,682
Gross unrealized gains	178	13
Gross unrealized losses	(51)	(300)
Estimated fair value	82,952	78,395
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	12,265	9,118
Gross unrealized gains	8	1
Gross unrealized losses	0	(2)
Estimated fair value	12,273	9,117
Corporate notes and bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	223,632	185,409
Gross unrealized gains	989	178
Gross unrealized losses	(79)	(457)
Estimated fair value	224,542	185,130
Foreign government bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	1,501	1,502
Gross unrealized gains	0	0
Gross unrealized losses	(3)	(11)
Estimated fair value	1,498	1,491
U.S. agency obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	3,000	15,912
Gross unrealized gains	0	2
Gross unrealized losses	0	(2)
Estimated fair value	3,000	15,912
U.S. treasury securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	276,655	243,119
Gross unrealized gains	326	78
Gross unrealized losses	(66)	(62)
Estimated fair value	$ 276,915	$ 243,135