Fair Value of Financial Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Fair Value of Financial Assets and Liabilities
Carrying amount of cash and cash equivalents and short term investments	$ 404,750	$ 172,792
Fair value of financial assets and liabilities
Marketable Securities	317,479	71,938
Corporate Debt Securities and Commercial Paper
Fair value of financial assets and liabilities
Marketable Securities	317,479	71,938
Recurring \| Estimated fair value
Fair value of financial assets and liabilities
Fair value of assets	403,594	171,362
Recurring \| Commercial Paper \| Estimated fair value
Fair value of financial assets and liabilities
Cash Equivalents	4,750
Recurring \| Corporate Debt Securities and Commercial Paper \| Estimated fair value
Fair value of financial assets and liabilities
Cash Equivalents		36,242
Marketable Securities	317,479	71,938
Recurring \| Money Market Funds \| Estimated fair value
Fair value of financial assets and liabilities
Cash Equivalents	81,365	63,182
Recurring \| Level 1 \| Estimated fair value
Fair value of financial assets and liabilities
Fair value of assets	81,365	63,182
Recurring \| Level 1 \| Money Market Funds \| Estimated fair value
Fair value of financial assets and liabilities
Cash Equivalents	81,365	63,182
Recurring \| Level 2 \| Estimated fair value
Fair value of financial assets and liabilities
Fair value of assets	322,229	108,180
Recurring \| Level 2 \| Commercial Paper \| Estimated fair value
Fair value of financial assets and liabilities
Cash Equivalents	4,750
Recurring \| Level 2 \| Corporate Debt Securities and Commercial Paper \| Estimated fair value
Fair value of financial assets and liabilities
Cash Equivalents		36,242
Marketable Securities	$ 317,479	$ 71,938

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).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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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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Cash includes currency on hand as well as demand deposits with banks or financial institutions. It also includes other kinds of accounts that have the general characteristics of demand deposits in that the customer may deposit additional funds at any time and effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid Investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means 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. Short-term investments, exclusive of cash equivalents, generally consist of marketable securities intended to be sold within one year (or the normal operating cycle if longer) and may include trading securities, available-for-sale securities, or held-to-maturity securities (if maturing within one year), as applicable.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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