Financial Instruments - Fair Value of Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands		Jan. 31, 2019	Jan. 31, 2018		Jan. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost		$ 793,932	$ 365,378
Gross Unrealized Gains		900	116
Gross Unrealized Losses		(1,238)	(2,033)
Fair Value		836,632	395,518
Cash Equivalents		71,327	27,466
Marketable Securities		749,482	353,289
Restricted cash		15,823	14,763	[1],[2]
Restricted cash	[1]				$ 12,734
Restricted Cash		15,823	14,763
Corporate Debt Securities | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost		363,860	186,506
Gross Unrealized Gains		534	116
Gross Unrealized Losses		(757)	(1,049)
Fair Value		363,637	185,573
Cash Equivalents		0	0
Marketable Securities		363,637	185,573
Restricted Cash		0	0
Foreign Government Debt [Member] | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost		7,965
Gross Unrealized Gains		36
Gross Unrealized Losses		0
Fair Value		8,001
Cash Equivalents		0
Marketable Securities		8,001
Restricted Cash		0
US Government Agencies Debt Securities | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost		69,114	47,229
Gross Unrealized Gains		17	0
Gross Unrealized Losses		(154)	(333)
Fair Value		68,977	46,896
Cash Equivalents		9,983	0
Marketable Securities		58,994	46,896
Restricted Cash		0	0
US Government Debt Securities | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost		315,329	131,643
Gross Unrealized Gains		208	0
Gross Unrealized Losses		(315)	(651)
Fair Value		315,222	130,992
Cash Equivalents		34,129	10,172
Marketable Securities		281,093	120,820
Restricted Cash		0	0
Money Market Funds [Member] | Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair Value		43,038	32,057
Cash Equivalents		27,215	17,294
Marketable Securities		0	$ 0
Asset-backed Securities | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost		37,664
Gross Unrealized Gains		105
Gross Unrealized Losses		(12)
Fair Value		37,757
Cash Equivalents		0
Marketable Securities		37,757
Restricted Cash		$ 0

[1] * As adjusted to reflect the impact of ASU 2016-18 and the full retrospective adoption of

[2] * As adjusted to reflect the impact of the full retrospective adoption of ASC 606