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Investments - Additional Information (Detail) - USD ($) | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Mar. 31, 2019 | Sep. 30, 2021 | Sep. 30, 2020 | Dec. 31, 2020
Schedule of Investments [Line Items] |  |  |  |
Available-for-sale debt securities, allowance for credit loss |  | $ 0 | $ 0 |
Purchase an available-for-sale debt securities with credit deterioration |  | 0 |  |
Transfer from investments | $ 3,400,000 |  |  |
Asset impairments |  |  | $ 65,000 |
Carrying value of investment |  | 800,000 |  | $ 800,000
Net Investment Gain (Loss) [Member] |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
Asset impairments |  |  |  | 2,600,000
Other Receivables [Member] |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
Note receivable, current | $ 4,300,000 | 4,300,000 |  | 4,300,000
Asset impairments |  | 0 |  |
Long-term Investments [Member] |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
Note receivable, noncurrent |  | $ 900,000 |  | $ 900,000
Investment [Member] | Issuer Concentration [Member] | Market Value of Total Investment Portfolio [Member] |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
Investment concentration risk percentage |  | 5.00% |  |

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