Financial Instruments and Fair Value - Schedule of Available-for-Sale Securities at Fair Value (Detail) - USD ($) $ in Thousands	Oct. 31, 2018	Jan. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 203,629	$ 101,986
Unrealized Gains	1	0
Unrealized Losses	(186)	(279)
Fair Value	203,444	101,707
Money market funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	9,472	13,788
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	9,472	13,788
Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6,004
Unrealized Gains	0
Unrealized Losses	0
Fair Value	6,004
Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	87,756	5,480
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	87,756	5,480
Corporate bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	70,169	53,175
Unrealized Gains	1	0
Unrealized Losses	(175)	(196)
Fair Value	69,995	52,979
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	9,547	11,048
Unrealized Gains	0	0
Unrealized Losses	(9)	(44)
Fair Value	9,538	11,004
U.S. government securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	20,681	18,495
Unrealized Gains	0	0
Unrealized Losses	(2)	(39)
Fair Value	$ 20,679	$ 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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