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