Financial Instruments and Fair Value - Schedule of Available-for-Sale Securities at Fair Value (Detail) - USD ($) $ in Thousands	Jan. 31, 2019	Jan. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 205,009	$ 101,986
Unrealized Gains	176	0
Unrealized Losses	(79)	(279)
Fair Value	205,106	101,707
Money market funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	8,914	13,788
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	8,914	13,788
Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,012
Unrealized Gains	0
Unrealized Losses	0
Fair Value	7,012
Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	68,233	5,480
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	68,233	5,480
Corporate bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	87,250	53,175
Unrealized Gains	170	0
Unrealized Losses	(60)	(196)
Fair Value	87,360	52,979
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	11,607	11,048
Unrealized Gains	3	0
Unrealized Losses	(6)	(44)
Fair Value	11,604	11,004
U.S. government securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	21,993	18,495
Unrealized Gains	3	0
Unrealized Losses	(13)	(39)
Fair Value	$ 21,983	$ 18,456

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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

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