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Fair Value Measurements Fair Value Measurements (Tables) | 3 Months Ended
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Mar. 31, 2022
Fair Value Disclosures [Abstract] |
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Table Text Block] | The following table summarizes the levels of fair value measurements of our financial assets:
| March 31, 2022 |  | December 31, 2021
(in thousands) | Total | Level 1 | Level 2 |  | Total | Level 1 | Level 2
Money market funds | $ | 579,096 |  | $ | 579,096 |  | $ | — |  |  | $ | 579,105 |  | $ | 579,105 |  | $ | — |
U.S. Treasury bills | 6,856 |  | 6,856 |  | — |  |  | 5,782 |  | 5,782 |  | — |
Municipal bonds | 25,754 |  | — |  | 25,754 |  |  | 26,009 |  | — |  | 26,009 |
Total | $ | 611,706 |  | $ | 585,952 |  | $ | 25,754 |  |  | $ | 610,896 |  | $ | 584,887 |  | $ | 26,009 |
Investments Classified by Contractual Maturity Date [Table Text Block] | As of March 31, 2022, the contractual maturities of the marketable securities in our portfolio were less than one year.
Debt Securities, Available-for-sale | The following is a summary of our available-for-sale marketable securities:
(in thousands) | Amortized Cost | Gross Unrealized Gains | Gross Unrealized Losses | Estimated Fair Value
March 31, 2022 |  |  |  |
U.S. Treasury bills | $ | 6,874 |  | $ | — |  | $ | (18) |  | $ | 6,856 |
Municipal bonds | 25,837 |  | — |  | (83) |  | 25,754 |
December 31, 2021 |  |  |  |
U.S. Treasury bills | $ | 5,783 |  | $ | — |  | $ | (1) |  | $ | 5,782 |
Municipal bonds | 26,017 |  | — |  | (8) |  | 26,009 |

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- Definition Tabular disclosure 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/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -URI http://asc.fasb.org/topic&trid=2196928 + Details
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- Definition Tabular disclosure of assets measured at fair value measured on a recurring or nonrecurring basis. Includes, but is not limited to, fair value measurements recorded and the reasons for the measurements, level within the fair value hierarchy in which the fair value measurements are categorized and transfers between levels 1 and 2. + References 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) -URI http://asc.fasb.org/extlink&oid=123874694&loc=d3e19207-110258 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=123874694&loc=d3e19207-110258 + Details
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