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Cash and Investments in Marketable Securities - Additional Information (Detail) - USD ($) | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2020 | Sep. 30, 2019 | Dec. 31, 2019
Cash and Investments in Marketable Securities [Line Items] |  |  |  |  |
Maximum maturity term for debt securities investment |  |  | 2 years |  |
Weighted average maturity term for debt securities investment (or less) |  |  | 1 year |  |
Available-for-sale securities, sold | $ 0 | $ 0 | $ 41,700,000 | $ 0 |
Accrued interest receivable | $ 5,400,000 |  | $ 5,400,000 |  | $ 6,500,000
Minimum |  |  |  |  |
Cash and Investments in Marketable Securities [Line Items] |  |  |  |  |
Long term investment maturity period |  |  | 1 year |  | 1 year
Maximum |  |  |  |  |
Cash and Investments in Marketable Securities [Line Items] |  |  |  |  |
Long term investment maturity period |  |  | 2 years |  | 2 years

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- Definition Carrying amount as of the balance sheet date of current interest earned but not received. Also called accrued interest or accrued interest receivable. For classified balance sheets, represents the current amount receivable, that is amounts expected to be collected within one year or the normal operating cycle, if longer. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.8) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
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- Definition Amount of cash inflow from sale 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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=121553693&loc=d3e26853-111562 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3151-108585 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 12 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3179-108585 + Details
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