Financial Instruments and Fair Value - Schedule of Cash Equivalents and Marketable Debt Securities (Detail) - USD ($) $ in Thousands	Jan. 31, 2019	Jan. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Total cash equivalents and marketable debt securities	$ 205,106	$ 101,707
Included in cash equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Total cash equivalents and marketable debt securities	40,245	13,788
Included in marketable debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total cash equivalents and marketable debt securities	$ 164,861	$ 87,919

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

Name:	us-gaap_AvailableForSaleSecuritiesDebtSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant