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Notes Payable - Components of Notes Payable (Detail) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Debt Instrument [Line Items] |  |
Notes payable | $ 303,473 | $ 326,376
Unsecured Revolving Lines of Credit [Member] |  |
Debt Instrument [Line Items] |  |
Notes payable | 183,473 | 186,376
Series A senior notes [Member] | 4.03% Series A senior notes due in 2018 [Member] |  |
Debt Instrument [Line Items] |  |
Notes payable | 20,000 | 40,000
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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- Definition Notes payable amount after reclassification of prepaid debt financing cost. + References No definition available. + Details
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Balance Type: | credit
Period Type: | instant

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Details
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- Details
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