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Marketable Securities - Additional Information (Details) - USD ($) | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2020 | Sep. 30, 2019 | Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Unrealized loss position for a period longer than twelve months | $ 0 |  | $ 0 |  |
Allowance for credit losses | 0 |  | 0 |  |
Impairment charges | 0 | $ 0 | 0 | $ 0 |
Marketable securities, due within one year | 20,457,000 |  | 20,457,000 |  | $ 34,874,000
Gross realized gain related to the sale of marketable securities | 0 | $ 0 | 35,000 | $ 0 |
Variable rate demand notes |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Marketable securities, due within one year | $ 5,600,000 |  | $ 5,600,000 |  |
Variable rate demand notes | Minimum |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Contractual maturities (in years) |  |  | 17 years |  |
Settlement basis period (in days) |  |  | 1 day |  |
Effective maturity period (in days) |  |  | 1 day |  |
Variable rate demand notes | Maximum |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Contractual maturities (in years) |  |  | 25 years |  |
Settlement basis period (in days) |  |  | 7 days |  |
Effective maturity period (in days) |  |  | 7 days |  |

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- Definition Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in next fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 320 -Section 50 -Paragraph 3A -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=120252992&loc=SL120269850-112803 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27198-111563 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563 + Details
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