Fair Value Measurements Fair Value Measurements (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Assets, Fair Value Disclosure [Abstract]
Money Market Funds	$ 551,213	$ 363,075
US Government Securities, at Carrying Value	10,531	34,728
Municipal bonds	23,998	0
Total	585,742	397,803
US Treasury Securities [Member]
Debt Securities, Available-for-sale [Abstract]
Available-for-sale Securities, Amortized Cost Basis	10,530	34,716
Available-for-sale Securities, Gross Unrealized Gain	1	13
Available-for-sale Securities, Gross Unrealized Loss	0	(1)
Fair Value	10,531	34,728
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Fair Value	10,531	34,728
Municipal Bond [Member]
Debt Securities, Available-for-sale [Abstract]
Available-for-sale Securities, Amortized Cost Basis	23,994
Available-for-sale Securities, Gross Unrealized Gain	8
Available-for-sale Securities, Gross Unrealized Loss	(4)
Fair Value	23,998
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Fair Value	23,998
Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Money Market Funds	551,213	363,075
US Government Securities, at Carrying Value	10,531	34,728
Municipal bonds	0	0
Total	561,744	397,803
Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Money Market Funds	0	0
US Government Securities, at Carrying Value	0	0
Municipal bonds	23,998	0
Total	$ 23,998	$ 0

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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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Amount of unrealized gain before deducting unrealized loss on investments in debt and equity securities classified as available-for-sale securities.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
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Amount of unrealized loss before deducting unrealized gain on investments in available-for-sale securities.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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-Section 50
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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
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-SubTopic 10
-Section 50
-Paragraph 2
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Debt (bills, notes or bonds) that are issued by the government of the United States which are 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. 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.

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