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Summary of Significant Accounting Policies - Fair Value Measurements (Details) - USD ($) $ in Millions | Dec. 31, 2021 | Dec. 31, 2020
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Level 1 |  |
Fair Value Measurements |  |
Carrying value of cash held in money market fund | $ 88.3 | $ 135.0
Level 2 |  |
Fair Value Measurements |  |
Assets measured at fair value levels 2 or 3 | 0.0 | 0.0
Liabilities measured at fair value levels 2 or 3 | 0.0 | 0.0
Level 3 |  |
Fair Value Measurements |  |
Assets measured at fair value levels 2 or 3 | 0.0 | 0.0
Liabilities measured at fair value levels 2 or 3 | $ 0.0 | $ 0.0

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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 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=123874694&loc=d3e19207-110258 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition Fair value of financial and nonfinancial obligations. + 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 + Details
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- Definition Investment in short-term money-market instruments (such as commercial paper, banker's acceptances, repurchase agreements, government securities, certificates of deposit, and so forth) which are highly liquid (that is, readily convertible to known amounts of cash) and so near their maturity that they present an insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify as cash equivalents by definition. Original maturity means an 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. + References No definition available. + Details
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