Notes Payable - Components of Notes Payable (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Notes payable	$ 326,376	$ 381,441
Unsecured Revolving Lines of Credit [Member]
Debt Instrument [Line Items]
Notes payable	186,376	221,441
Series A senior notes [Member] | 4.03% Series A senior notes due in 2018 [Member]
Debt Instrument [Line Items]
Notes payable	40,000	60,000
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	$ 60,000