Fair Value Measurements Fair Value Measurements (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	$ 454,477	$ 551,213
US Government Securities, at Carrying Value	11,074	10,531
Municipal bonds	21,623	23,998
Total	487,174	585,742
Debt Securities, Available-for-sale, Amortized Cost, Fiscal Year Maturity [Abstract]
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost	31,573
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost	1,132
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Fair Value	31,566
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value	1,131
Debt Securities, Available-for-sale, Amortized Cost	32,705	34,524
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	0	9
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	8	4
Debt Securities, Available-for-sale	32,697	34,529
US Treasury Securities [Member]
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Debt Securities, Available-for-sale, Amortized Cost	11,074	10,530
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	0	1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0	0
Debt Securities, Available-for-sale	11,074	10,531
Municipal Bonds
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Debt Securities, Available-for-sale, Amortized Cost	21,631	23,994
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	0	8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	8	4
Debt Securities, Available-for-sale	21,623	23,998
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	454,477	551,213
US Government Securities, at Carrying Value	11,074	10,531
Municipal bonds	0	0
Total	465,551	561,744
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	0	0
US Government Securities, at Carrying Value	0	0
Municipal bonds	21,623	23,998
Total	$ 21,623	$ 23,998

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, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on 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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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on 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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Amortized cost 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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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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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

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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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