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Divestitures - Additional Information (Detail) - USD ($) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2020 | Sep. 30, 2020 | Dec. 31, 2019 | Jun. 30, 2019 | Mar. 31, 2019 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |
Gain (loss) on divestitures and sale of investments | $ 403,000 | $ (9,000) | $ (14,000) | $ (1,619,000) | $ 20,054,000 | $ 394,000 | $ 18,421,000 |
Other Sales [Member] |  |  |  |  |  |  |  |
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |
Gain (loss) on divestitures and sale of investments |  |  |  |  |  | $ 0 | $ 18,400,000 | $ 0

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