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Notes Payable - Components of Notes Payable (Detail) - USD ($) $ in Thousands | Dec. 31, 2015 | Sep. 30, 2015 | Jun. 30, 2015 | Mar. 31, 2015 | Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014
--- | --- | --- | --- | --- | --- | --- | --- | ---
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Notes payable | $ 381,441 | $ 382,113 | $ 337,177 | $ 320,923 | $ 322,478 | $ 322,280 | $ 307,000 | $ 288,081
Unsecured Revolving Lines of Credit [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Notes payable | 221,441 |  |  |  | 202,478 |  |  |
Series A senior notes [Member] | 4.03% Series A senior notes due in 2018 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Notes payable | 60,000 |  |  |  | 80,000 |  |  |
Series B Senior Notes | 3.68% Series B senior notes due in 2021 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Notes payable | 40,000 |  |  |  | $ 40,000 |  |  |
Series C Senior Notes | 3.84% Series C senior notes due in 2022 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Notes payable | $ 60,000 |  |  |  |  |  |  |

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