Financial Instruments (Summary of Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Assets
Total cash and cash equivalents	$ 81,611	$ 29,627
Total investments	237,203	19,472
Short-term restricted cash: Cash	836	300
Long-term restricted cash: Cash	3,500	4,000
Total assets measured at fair value	323,150	53,399
Cash and money market funds [Member]
Assets
Total cash and cash equivalents	43,040	18,644
Commercial paper [Member]
Assets
Total cash and cash equivalents	32,537	10,983
U.S. government and agency securities [Member]
Assets
Total cash and cash equivalents	170
U.S. Treasury security [Member]
Assets
Total cash and cash equivalents	5,864
Commercial paper, not included with cash and cash equivalents [Member]
Assets
Total investments	112,644	16,971
Corporate debt securities [Member]
Assets
Total investments	17,456	2,501
U.S. government & agency securities, not included with cash and cash equivalents [Member]
Assets
Total investments	107,103
Level 1 [Member]
Assets
Total cash and cash equivalents	43,040	18,644
Short-term restricted cash: Cash	836	300
Long-term restricted cash: Cash	3,500	4,000
Total assets measured at fair value	47,376	22,944
Level 1 [Member] \| Cash and money market funds [Member]
Assets
Total cash and cash equivalents	43,040	18,644
Level 2 [Member]
Assets
Total cash and cash equivalents	38,571	10,983
Total investments	237,203	19,472
Total assets measured at fair value	275,774	30,455
Level 2 [Member] \| Commercial paper [Member]
Assets
Total cash and cash equivalents	32,537	10,983
Level 2 [Member] \| U.S. government and agency securities [Member]
Assets
Total cash and cash equivalents	170
Level 2 [Member] \| U.S. Treasury security [Member]
Assets
Total cash and cash equivalents	5,864
Level 2 [Member] \| Commercial paper, not included with cash and cash equivalents [Member]
Assets
Total investments	112,644	16,971
Level 2 [Member] \| Corporate debt securities [Member]
Assets
Total investments	17,456	$ 2,501
Level 2 [Member] \| U.S. government & agency securities, not included with cash and cash equivalents [Member]
Assets
Total investments	$ 107,103

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Short-Term Restricted Cash Fair Value Disclosure

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- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
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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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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
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