Investments and Fair Value Measurements (Details) - USD ($) $ in Thousands	Apr. 30, 2020	Jan. 31, 2020
Fair Value Measurements
U.S. treasury securities, corporate bonds and commercial paper	$ 834,067	$ 1,005,146
Estimate of Fair Value Measurement \| Recurring basis
Fair Value Measurements
Money market funds	329,321	138,999
Other	2,000	2,000
Assets:
Cash and cash equivalents	329,321	147,034
Investments, current	834,067	976,508
Investments, non-current	2,000	22,603
Total	1,165,388	1,146,145
Estimate of Fair Value Measurement \| Recurring basis \| Level 1
Fair Value Measurements
Money market funds	329,321	138,999
Other	0	0
Estimate of Fair Value Measurement \| Recurring basis \| Level 2
Fair Value Measurements
Money market funds	0	0
Other	0	0
Estimate of Fair Value Measurement \| Recurring basis \| Level 3
Fair Value Measurements
Money market funds	0	0
Other	2,000	2,000
US Treasury Securities \| Estimate of Fair Value Measurement \| Recurring basis
Fair Value Measurements
U.S. treasury securities, corporate bonds and commercial paper	730,619	875,180
US Treasury Securities \| Estimate of Fair Value Measurement \| Recurring basis \| Level 1
Fair Value Measurements
U.S. treasury securities, corporate bonds and commercial paper	0	0
US Treasury Securities \| Estimate of Fair Value Measurement \| Recurring basis \| Level 2
Fair Value Measurements
U.S. treasury securities, corporate bonds and commercial paper	730,619	875,180
US Treasury Securities \| Estimate of Fair Value Measurement \| Recurring basis \| Level 3
Fair Value Measurements
U.S. treasury securities, corporate bonds and commercial paper	0	0
Corporate Bonds \| Estimate of Fair Value Measurement \| Recurring basis
Fair Value Measurements
U.S. treasury securities, corporate bonds and commercial paper	103,448	124,972
Corporate Bonds \| Estimate of Fair Value Measurement \| Recurring basis \| Level 1
Fair Value Measurements
U.S. treasury securities, corporate bonds and commercial paper	0	0
Corporate Bonds \| Estimate of Fair Value Measurement \| Recurring basis \| Level 2
Fair Value Measurements
U.S. treasury securities, corporate bonds and commercial paper	103,448	124,972
Corporate Bonds \| Estimate of Fair Value Measurement \| Recurring basis \| Level 3
Fair Value Measurements
U.S. treasury securities, corporate bonds and commercial paper	0	0
Commercial Paper \| Estimate of Fair Value Measurement \| Recurring basis
Fair Value Measurements
U.S. treasury securities, corporate bonds and commercial paper	0	4,994
Commercial Paper \| Estimate of Fair Value Measurement \| Recurring basis \| Level 1
Fair Value Measurements
U.S. treasury securities, corporate bonds and commercial paper	0	0
Commercial Paper \| Estimate of Fair Value Measurement \| Recurring basis \| Level 2
Fair Value Measurements
U.S. treasury securities, corporate bonds and commercial paper	0	4,994
Commercial Paper \| Estimate of Fair Value Measurement \| Recurring basis \| Level 3
Fair Value Measurements
U.S. treasury securities, corporate bonds and commercial paper	$ 0	$ 0

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Investments, Noncurrent, Fair Value Disclosure

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Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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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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Fair value portion 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. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Investment in short-term money-market instruments (such as commercial paper, banker's acceptances, repurchase agreements, government securities, certificates of deposit, and so forth) which are highly liquid (that is, readily convertible to known amounts of cash) and so near their maturity that they present an insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify as cash equivalents by definition. Original maturity means an original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

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Fair value portion of other assets.

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