Fair Value Measurements - Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Financial Assets
Marketable securities	$ 511,774	$ 238,183
U.S. Treasuries
Financial Assets
Marketable securities	180,373	71,368
Commercial paper
Financial Assets
Marketable securities	41,904
Corporate equity securities
Financial Assets
Equity securities		3,667
Corporate notes/bonds
Financial Assets
Marketable securities	42,170
Money market funds
Financial Assets
Marketable securities	139,682	230,201
Recurring
Financial Assets
Total financial assets	515,651	462,426
Recurring \| U.S. Treasuries
Financial Assets
Cash and cash equivalents		7,982
Marketable securities	180,373	63,386
Recurring \| Government agency securities
Financial Assets
Marketable securities	107,645	155,571
Recurring \| Commercial paper
Financial Assets
Marketable securities	41,904
Recurring \| Corporate equity securities
Financial Assets
Restricted cash		3,667
Recurring \| Corporate notes/bonds
Financial Assets
Marketable securities	42,170
Recurring \| Money market funds
Financial Assets
Cash and cash equivalents	139,682	230,201
Restricted cash	3,877	1,619
Recurring \| Level 1
Financial Assets
Total financial assets	323,932	458,759
Recurring \| Level 1 \| U.S. Treasuries
Financial Assets
Cash and cash equivalents		7,982
Marketable securities	180,373	63,386
Recurring \| Level 1 \| Government agency securities
Financial Assets
Marketable securities		155,571
Recurring \| Level 1 \| Money market funds
Financial Assets
Cash and cash equivalents	139,682	230,201
Restricted cash	3,877	1,619
Recurring \| Level 2
Financial Assets
Total financial assets	191,719	3,667
Recurring \| Level 2 \| Government agency securities
Financial Assets
Marketable securities	107,645
Recurring \| Level 2 \| Commercial paper
Financial Assets
Marketable securities	41,904
Recurring \| Level 2 \| Corporate equity securities
Financial Assets
Restricted cash		$ 3,667
Recurring \| Level 2 \| Corporate notes/bonds
Financial Assets
Marketable securities	$ 42,170

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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Cost of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). Excludes equity method investment and investment in equity security without readily determinable fair value.

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Amount of cash and cash equivalents restricted as to withdrawal or usage. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, 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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