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