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Notes Payable - Schedule of Future Minimum Payments under Unsecured Revolving Lines of Credit and 3.68% Senior Notes and 3.84% Senior Notes (Detail) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Debt Instrument [Line Items] |  |
Notes payable | $ 222,771 | $ 293,459
Notes Payable to Banks and Others [Member] |  |
Debt Instrument [Line Items] |  |
2021 | 40,000 |
2022 | 60,000 |
2025 | 122,771 |
Notes payable | $ 222,771 |

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