Investments and Fair Value Measurements - Amortized Cost to Fair Value Reconciliation (Details) - USD ($) $ in Thousands	Apr. 30, 2020	Jan. 31, 2020
Debt Securities, Available-for-sale
Amortized Cost	$ 828,876	$ 1,003,896
Unrealized Gains	5,192	1,273
Unrealized Losses	(1)	(23)
Fair Value	834,067	1,005,146
Due within one year	834,067
Cash and Cash Equivalents \| US Treasury Securities
Debt Securities, Available-for-sale
Amortized Cost		8,035
Unrealized Gains		0
Unrealized Losses		0
Fair Value		8,035
Investments, Current \| US Treasury Securities
Debt Securities, Available-for-sale
Amortized Cost	726,104	866,578
Unrealized Gains	4,515	590
Unrealized Losses	0	(23)
Fair Value	730,619	867,145
Investments, Current \| Corporate Bonds
Debt Securities, Available-for-sale
Amortized Cost	102,772	103,848
Unrealized Gains	677	521
Unrealized Losses	(1)	0
Fair Value	$ 103,448	104,369
Investments, Current \| Commercial Paper
Debt Securities, Available-for-sale
Amortized Cost		4,991
Unrealized Gains		3
Unrealized Losses		0
Fair Value		4,994
Investments, Non-current \| Corporate Bonds
Debt Securities, Available-for-sale
Amortized Cost		20,444
Unrealized Gains		159
Unrealized Losses		0
Fair Value		$ 20,603

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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Amortized cost 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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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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