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Notes Payable - Components of Notes Payable (Detail) - USD ($) $ in Thousands | Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017
--- | --- | --- | --- | --- | --- | --- | --- | ---
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Notes payable | $ 298,603 |  |  |  | $ 303,473 |  |  |
Unamortized debt issuance cost | (39) |  |  |  | (59) |  |  |
Notes payable, net | 298,564 | $ 309,006 | $ 314,860 | $ 300,595 | 303,414 | $ 323,117 | $ 330,287 | $ 323,483
Unsecured Revolving Lines of Credit [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Notes payable | 198,603 |  |  |  | 183,473 |  |  |
Series A senior notes [Member] | 4.03% Series A senior notes due in 2018 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Notes payable | 0 |  |  |  | 20,000 |  |  |
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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