Fair Value Measurements - Assets and liabilities measured at fair value on a recurring basis (Details) - USD ($) $ in Thousands	Jan. 31, 2020	Jan. 31, 2019
Available-for-sale securities
Amortized Cost	$ 818,970	$ 851,534
Gross Unrealized Gains	1,204	400
Gross Unrealized Losses	(82)	(142)
Estimated Fair Value	820,092	851,792
Cash	75,800	81,400
Fair Value, Inputs, Level 2
Available-for-sale securities
Amortized Cost		501,471
Gross Unrealized Gains		400
Gross Unrealized Losses		(142)
Estimated Fair Value		501,729
Cash equivalents \| Money market funds \| Fair Value, Inputs, Level 1
Available-for-sale securities
Amortized Cost	165,424	350,063
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	165,424	350,063
Cash equivalents \| Commercial paper \| Fair Value, Inputs, Level 2
Available-for-sale securities
Amortized Cost		76,828
Gross Unrealized Gains		0
Gross Unrealized Losses		(11)
Estimated Fair Value		76,817
Cash equivalents \| Corporate notes and bonds \| Fair Value, Inputs, Level 2
Available-for-sale securities
Amortized Cost		2,998
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Estimated Fair Value		2,998
Cash equivalents \| U.S. government agency securities \| Fair Value, Inputs, Level 2
Available-for-sale securities
Amortized Cost		6,491
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Estimated Fair Value		6,491
Available-for-sale securities
Available-for-sale securities
Estimated Fair Value	654,668
Available-for-sale securities \| Fair Value, Inputs, Level 2
Available-for-sale securities
Amortized Cost	653,546
Gross Unrealized Gains	1,204
Gross Unrealized Losses	(82)
Estimated Fair Value	654,668
Available-for-sale securities \| Commercial paper \| Fair Value, Inputs, Level 2
Available-for-sale securities
Amortized Cost	14,919	86,655
Gross Unrealized Gains	7	4
Gross Unrealized Losses	(1)	(21)
Estimated Fair Value	14,925	86,638
Available-for-sale securities \| Corporate notes and bonds \| Fair Value, Inputs, Level 2
Available-for-sale securities
Amortized Cost	372,844	287,496
Gross Unrealized Gains	891	389
Gross Unrealized Losses	(31)	(105)
Estimated Fair Value	373,704	287,780
Available-for-sale securities \| U.S. Treasury securities \| Fair Value, Inputs, Level 2
Available-for-sale securities
Amortized Cost	90,697	4,982
Gross Unrealized Gains	153	0
Gross Unrealized Losses	(1)	(1)
Estimated Fair Value	90,849	4,981
Available-for-sale securities \| U.S. government agency securities \| Fair Value, Inputs, Level 2
Available-for-sale securities
Amortized Cost	175,086	36,021
Gross Unrealized Gains	153	7
Gross Unrealized Losses	(49)	(4)
Estimated Fair Value	$ 175,190	$ 36,024

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 currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Excludes cash and cash equivalents within disposal group and discontinued operation.

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