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Notes Payable - Components of Notes Payable (Detail) - USD ($) $ in Thousands | Dec. 31, 2020 | Sep. 30, 2020 | Jun. 30, 2020 | Mar. 31, 2020 | Dec. 31, 2019 | Sep. 30, 2019 | Jun. 30, 2019 | Mar. 31, 2019
--- | --- | --- | --- | --- | --- | --- | --- | ---
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Notes payable | $ 222,771 |  |  |  | $ 293,459 |  |  |
Unamortized debt issuance cost | (17) |  |  |  | (28) |  |  |
Notes payable, net | 222,754 | $ 249,980 | $ 272,149 | $ 291,544 | 293,431 | $ 301,469 | $ 301,878 | $ 289,464
Unsecured Revolving Lines of Credit [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Notes payable | 122,771 |  |  |  | 193,459 |  |  |
Series B senior notes [Member] | 3.68% Series B senior notes due in 2021 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Notes payable | 40,000 |  |  |  | 40,000 |  |  |
Series C senior notes [Member] | 3.84% Series C senior notes due in 2022 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Notes payable | $ 60,000 |  |  |  | $ 60,000 |  |  |

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