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Investments - Carrying Value and Amortized Cost of Company's Investments Summarized by Major Security Type (Parenthetical) (Detail) - Maximum - USD ($) $ in Thousands | 9 Months Ended | 12 Months Ended
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Sep. 30, 2021 | Dec. 31, 2020
US Government Sponsored Enterprises Debt Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Unrealized gain (loss) on securities | $ 500 |
Corporate debt securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Unrealized gain (loss) on securities |  | $ (500)

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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 Amount of unrealized gain (loss) recognized in the income statement on unsold other securities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=123570139&loc=d3e3602-108585 + Details
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